I am getting an error that I can't approve a bid as there is an approved overlapping bid, what does that mean?

The reason that you are getting this is that that agency candidate is already approved for another shift that overlaps with the one that you are currently trying to approve. You can see this in the summary of their neighbouring shifts that pops down or alternatively on their profile. If you still want to approve them for this shift then you would either need to decline them for the original shift, or adjust the timings of one of the shifts so they don’t overlap.