Steam – “This product is not compatible with macOS 10.15 Catalina” message on older macOS

I know this sounds odd, but try uninstalling it, and cleaning your registry with a program like CC Cleaner. Go through your computer and manually delete anything FO3 related, including the folder for it in 'My Documents'. Check the install folder after you remove it from Steam, and manually delete what is left in there as well. After reinstalling, load the game with no DLC checked, make a character, play through up until Megaton or so, then save. Reboot your game, check all of the DLC and load the game back up.

It's someone trying to scam you.

There is no way to send Steam Wallet credit in the trade system. Anyone suggesting otherwise is looking to take advantage of you.