Cannot complete Erase Free Space

you appear to have more than enough space to do this, but if your putting files in the trash and erasing them you are reclaiming that space.

the question is why are you doing this. Unless you need to securely delete files so there is no chance to recover them I suggest you read this

https://support.apple.com/en-us/HT201949

if this is what you need to do and it's not working I would disconnect all external devices, then boot into safe mode, then try to erase free space.

if you don't empty the trash it wont reclaim any space when you "trash" the file until you empty it out, it's essentially a folder. You probably know that but not everyone does so I thought I'd ask.

Do a backup, preferable 2 separate ones on 2 drives. Boot to the Recovery Volume (command - R on a restart or hold down the option/alt key during a restart and select Recovery Volume). See if you can do the erase there.