How to remove Filevault encryption within Recovery Mode in Catalina?

Disk Utility itself cannot disable FileVault. One needs to use the Security & Privacy preference panel to enable or disable FileVault. I have no recollection of controlling FileVault using Disk Utility in Recovery Mode. Have you checked the Utilities menu in the screen menubar? I don't think you can evoke System Preferences while in Recovery.

If you can boot the computer normally or Use safe mode to isolate issues with your Mac - Apple Support or Playing Safe- what does Safe mode do?, then you can use Security & Privacy to disable FileVault. Keep in mind that decrypting the disk takes quite a long time to complete.

My apologies. I meant starting up the computer or re-starting. You can follow these instructions with which you should be familiar:

Boot from the Recovery HD

1. Restart the computer.
2. Immediately, at or before the chime, press and hold down the "COMMAND" and "R" keys together.
3. Release the keys when the Apple logo and progress bar appear.
4. Upon completion the Utility Menu should appear.

The Utility Menu appears on a gray screen. There is a menubar on the screen. The Utility Menu has four selections: Restore from Time Machine backup; Install macOS; Get help; and, open Disk Utility. In the lower right corner of the Utility Menu is a button labeled, Continue. You can open any of the options either by double-clicking on the option or by selecting the option and clicking on the Continue button.

So, to do what you asked:

Install El Capitan or Later by Erasing the Disk

If possible, back up your files.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
5. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

If you don't want to erase the disk, here is how you reinstall macOS over an existing system without erasing the disk:

Reinstall El Capitan or Later Without Erasing Drive

Please be sure you back up, if possible.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the side list.
4. Click on the First Aid button in Disk Utility's toolbar. Wait until the Done button activates, then click on it.
5. Quit Disk Utility and return to the Utility Menu.
6. Select Install OS X and click on the Continue button.

When you boot from the Recovery HD, you will be presented with the Utility Menu on a medium gray screen. The menubar is at the top of the screen. If that is not the case for you, then the Recovery HD is corrupted.

If the Recovery HD is corrupted, then most likely you could not boot the machine from the Recovery HD. If you are able to boot from the Recovery HD but cannot disable FileVault, then you probably have some other disk-related problem and will have to reformat the disk while in Recovery HD.

Reinstall the OS from scratch and when the

part that says “active FireVault”, uncheck it this time unless you’re a multinational multi-million dollar corporation. Only about 5% or less of users need it.

You need to do what I posted previously - Erase and Install. You no longer can recover your files unless you have a backup.