Hi Grace!
I was experiencing a similar issue (Error 500 when trying to sync experiment from Builder to Pavlovia) and ended up following the advice listed here: HTTP 500 error on initial git push.
Let me know if you have trouble following those steps! Just type "cd " (with the space) into terminal, drag the hidden .git file into terminal, press enter, and then use commands listed in this solution:
git config http.postBuffer 524288000
followed by:
git push -u origin master