You would need to insert the current field “My Completed Course” into your automation for the field “My Completed Course” (to grab the old values for “My Completed Course”), then type a comma, then add the new value.
#1. You can only put a single Record ID in the “Record ID” step. You are currently trying to put in an entire list of Record ID’s.
#2. Then, for the multi-select field that you’re trying to update, you need to call the “Names” value of that very same multi-select field, then put a comma, and then put the new value in (whatever that new value might be).
See my screenshot below for an example of how this could look: