OK I just checked, on the keyboard I had to press the clipboard icon to paste. Long press didn't initially work for some reason. After I pasted from the keyboard, long press to paste worked. Weird, must be some android quirk.

Edit: This might help (sorry for the reddit link): https://www.reddit.com/r/GalaxyS21/comments/r0m6ub/clipboard_no_longer_accessible/i15sk5n/

OP:

Typically, and prior to the Android 12 update, if I long-pressed or double pressed a text field, an option would come up for paste and clipboard. Now, however, the clipboard option is not showing up. I can't seem to figure out how to re-enable this option, and as someone who used it frequently, I'd really like to have it back.

Reply:

Figured it out finally! Open keyboard to type and click on the little 3 dots upper right and it brings up a menu with clipboard as an option. This was frustrating!