split cell button not working

wheppner 

Can someone point me in the right direction of how to fix this?

I'm not 100% positive about this, but am reasonably confident: the "split cell" capability that you're referring to only applies to cells that have previously been merged. In other words, it's there to restore "normal" cell alignment; it's not there to take an otherwise normal cell and turn it into two cells. That would wreak havoc in formulas, for one thing.

Assuming that's correct--and as I said I'm reasonably confident that it is--how about backing up and giving us an idea of what you're trying to do: is it making a heading section more detailed, for example? 

If it's possible, perhaps you could post a copy of the spreadsheet in question on OneDrive or GoogleDrive, with a link pasted here that grants access to it. Then accompany that with the clear description of what you are wanting to do that has led you to want to split a cell or two.