Glide, glide, glide…
If you ask me, glide tables should be in staging
I’m not even going to describe 
How very extremely immensely annoying for me. I was just implementing Glide Sheets into my app. Can you please make my day by fixing this?
I think that’s because you don’t have any columns in that table to build a relation.
Oh my god… I’m such a big idiot 
That might be the reason…
But what I was going to do is build a relation referring to a Google Sheet. Is this possible?
Yes — you can relate columns from your Glide Table to columns in your Google Sheet.
I’m thinking you may be trying to build a full copy of a GSheet and make it a Glide Table. That currently isn’t possible, but you could add columns to your Glide Table, then create relations and perform lookups to bring over the GSheet information.