Is there a quick way to fix overlap errors?

Hey everyone,

I've been trying to run Flow simulation on a car that we're building. Unfortunately, I still have gotten errors from an edited file my colleague sent me. I know that it could be a rather simple solution, however, I've tried hours upon hours to solve the problem. I can't run a diagnostics tool or merge the parts. I've even considred trying to mold the whole assembly into one part. Can anyone share what could work?

I have my screenshot attached.

thanks in advance,

Luke