Hi Gord, is it these 2 pieces here that you were trying to join? :



If so the reason you can't join those is the larger block is a closed solid, it does not have any unjoined edges in it. You can only Join pieces that are touching each other at unjoined edges.

You would need to delete a face of that closed solid to make it have unjoined edges and then that's where it can be joined if it matches close enough.

But also there appears to be a really skinny slivery extra surface in this area here:



That will also probably mess up joining, that degenerate surface will probably need to be removed for it to be able to join ok.

- Michael