Generate transforms only working on default install (multi-site)
From Brandon Kelly on Discord:

If you go to the Assets index page and change the site to your second
site. Are you able to see all the same assets there? (Specifically
whatever ones appear to be causing this error on your second site)

They weren't listed.

Had a feeling that would be the case. That's your problem. If you run
php craft resave/assets, does that fix it?

Yes it did! Thanks :-)