install El Capitan on a fresh SSD drive to be put in an iMac 7,1 afterwards... using my MacBook Pro (MBP) with Catalina in order to install El Capitan on the SSD( connected to my MBP through a USB-C enclosure) But it don't work

Okay let's get a bit of clarity.

If you want to download El Capitan you can get it here,

How to upgrade to OS X El Capitan – Apple Support

you then go to Section 4 and download InstallMacOSX.dmg

which downloads to your Downloads folder.

You then open that to get the InstallMacOSX.pkg.

These are things you have done on your MBP with Catalina,

you can do these two things on any mac.

BUT here is where it goes wrong as this next section can only be done on a mac

that is capable of running El Capitan, therefore any mac that came pre-installed

with a newer OS like macOS Sierra and newer will refuse to do this next bit.

Double-clicking on InstallMacOSX.pkg will open the installation window as

shown in your screenshot, this does not install El Capitan but will create the

Install OS X El Capitan.app in your Applications folder.

You then use the Install OS X El Capitan.app in the Applications folder and the createinstallmedia

command in Terminal to create the USB installer. (This part can be done on any mac.)

So let's just recap, you can do some of the procedures on your MacBook Pro

except for one crucial one. (Well done Apple really well thought out)

You really need to do this on a mac that is capable of running El Capitan,

that includes any macs that are are eligible to install El Capitan but may be

running older OS's like Lion, ML, Mavericks or Yosemite. A mac that is capable

of running or at one time had been running El Capitan but has been upgraded

to a newer version of mac OS can be used to do this.

Any macs that came preinstalled with a newer macOS like Sierra, High Sierra, Mojave

or Catalina will refuse to do the creation of the Install OS X El Capitan.app.

Hope that clears things up a bit.

Catalina will not allow you to download El Capitan nor run the installer should you already have it. If you have the El Capitan installer, then you need to create a bootable USB flash drive installer. See the following instructions. I have put the El Capitan in bold. Below it is the command line you need to create the bootable USB flash drive installer.

Bootable USB Installers for Mavericks through Catalina

First, review this introductory article: Create a bootable installer for macOS. Second, read this How To outline for creating a bootable macOS installer. Simply use the Terminal command by copying and pasting the command line for the version of macOS from the list below. You will need an 8GB or larger USB flash drive that has been partitioned and formatted for use with OS X as well as an installer for the desired macOS version that you have previously downloaded from the App Store.

Drive Partition and Format

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the w/type and size info) from the side list.
3. Click on the Erase button in the Disk Utility toolbar. Name the drive, "MyVolume". <---- IMPORTANT!
4. In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button and wait for the Done button to activate. When it does click on it.
6. Quit Disk Utility.

Create Installer

Open the Terminal in the Utilities' folder. Choose the appropriate command line (begins with "sudo") depending upon what OS X installer you want. Paste that entire command line from below at the Terminal's prompt:

Command for macOS Catalina:

sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Command for macOS Mojave:

sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

*Command for macOS High Sierra:

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Command for macOS Sierra:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app

Command for El Capitan:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

Command for Yosemite:

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app

Command for Mavericks:

sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Mavericks.app

Press RETURN. You will be asked for your admin password. It will not echo to the Terminal window. Then press RETURN again. Wait for the return of the Terminal prompt signifying the process has completed. It takes quite some time to finish. Be patient.

The bootable installer can be used to install the selected version of macOS. Shut the computer down. Insert your flash drive into a USB port. Start the computer like so:

Boot Using OPTION key

1. Restart the computer.
2. Immediately after the chime press and hold down the  "OPTION" key.
3. Release the key when the Boot Manager screen appears.
4. Select the disk icon for the USB flash drive.
5. Click on the arrow button under the disk icon.

The installer app is in the disk image. Double-click on the .dmg to open it, then drag the content (installer app) onto your Desktop. This is the Install OS X El Capitan.app. If you find a .pkg file, then double-click on it to install the installer in your Applications folder. The is the file used by the command line.

You should be able to get that far on your Catalina machine because you will not be opening the installer. You will just use it to make the USB flash drive. The instructions I provided have been proven both by me and other users, so it should work for you.