command not found. error

As P. Phillips said: did you download the installer?

Go to the Applications folder. Is "Install macOS Big Sur" in there?

Apparently, not...

The "command not found" message is pretty clear - there is no command at that location.

You need to

1) download the installer;

2) when it starts, quit it.

Note: If you download and then let the installer run and install Big Sur, it will delete itsef when done.

You have to either quit it before installing, or make a copy of it somewhere else for safekeeping.

Firstly, this is predicated on the Full Installer of Big Sur has been downloaded Successfully and is present in the Applications Folder and no where else.

Secondly - there is a USB Drive attached with at least 16 GB Capacity and noting on the USB Drive

Thirdly - the USB Drive is formatted as HFS Journaled and GUID Partition Map

Fourthly - the USB Drive has the Name " Untitiled "

Open the Terminal Application and copy and paste this command without the Quotations Marks

" sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled "

You will be asked for Computer Password which you will have to be manually input - there will be no echo to see the password typed.

The process from above command will take some time to perform - so patience is needed.

In addition to what the other contributors have mentioned you did not provide the proper path since you did not properly "escape" the spaces in the path using a backslash "\". Check out the command example provided by @P. Phillips.