Sims won't workout in fitness attire

Well, have you tried to recreate this issue on a vanilla game with the mods removed?  In order to test to see if its mod-related, you'll need to remove your mods folder (to your desktop) and delete the localthumbnailcache from your sims 4 file on your computer.  Start a new game, bring sims to the gym and test again.  also, before a mod tells you, you should actually locate the patch number (1.58.69.1010) rather than say "current patch" so that people don't get confused if there's a hotfix/update, as some known issues are several patches old.

if you find that the error is mod/cc related, you should try the 50/50 method to determine which mod is causing the issue.