Insert column restricted

[Ginger_C]

I have an Excel spreadsheet created and update even a year ago (July 2020) and I am trying to insert a cl=column, but with new Office 365 the message I get is 

"Microsoft Excel can't insert new cells because it would push non-empty cells off the end of the worksheet. These no-empty cells might appear empty but have blank values , some formatting or a formula.  Delete enough rows or columns to make room for what you want to insert and then try again."

Ugh- I have tried everything.  This is a HUGE spreadsheet.  Any suggestiongs?   Something hidden I cannot see.  

[#2]

@Ginger_C 

Could you please clarify a bit what a cl=column exactly means? If something like

=N5=A:A

added to M5 that definitely doesn't work. In previous version of Excel formula compares N5 with A5 (value in current row) and returns the result. So called implicit intersection.

It doesn't work in Excel which support dynamic arrays. Excel compares entire one million values in column A with N5 and tries to return one million of resulting values starting from M5. Not enough space for that.

Above is only my guess of what do you try to do exactly.