Problem inserting cells on a worksheet

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I have one worksheet in an excel 2016 workbook that only offers insert row or column but not insert cell. All other sheets are fine. The affected sheet has no filters and I have deleted all columns and rows beyond the last occupied cell. Also sheets are not protected

Any ideas what on this sheet may be causing the issue?

I have one idea, and I'm pretty sure it's the reason: you can't insert a cell. That's why it's not allowed.

Now, think about it: if you inserted a cell now--just a cell--in, let's assume, what is now cell B23. What would your new cell be called? How would you reference it in a formula? Would that one cell that is now two become B23a and B23b (like one may see from time to time on houses that are converted to duplexes)? 

In other words, what you're asking doesn't make sense in connection with how spreadsheets work.

So let's back up and ask what it is that causes you to want to insert a cell, to turn a single cell into two. What are you trying to accomplish that has led you to think "I need to insert a cell"? There most certainly will be a way to accomplish that underlying goal and do so within the boundaries of what's possible in Excel. So let's start there.