Why when trying to insert a new (Spreadsheet) row does the error message: Filled cells cannot be shifted beyond the sheet....mean?

Have spent last 2 hours trying to figure out why the icons on Spreadsheet are not working appropriately or not at all. I need to add rows…but trying to insert a row only brings up error message: Filled cells cannot be shifted beyond the sheet…What ever does that mean? It’s a simple task that usually causes no problems whatsoever?? Also the ‘Borders’ icon not working properly. It won’t do as I request, allow me to highlight a cell in colour.
I have never had a problem with Libre Office before but I am trying to do a friend’s book work for his Accountant & need Libre to be working as it usually does. PLEASE HELP. Thankyou… Mojo

(This is about shifting rows on insertions. Please don’t mix up many questions in one thread.)

Inserting a row cannot increase the maximum number of rows per sheet (2^20). The rows above which new rows are inserted must be shifted down getting higher row numbers, and they are.
However, if there is a used cell anywhere that cannot be shifted down by the needed distance within the acceptable range of row numbers, the software will deny action - and you should be gald about this.
Use the modified key ‘Ctrl’+‘End’ and you will go to the cell where the bottommost row and the rightmost column containing a used (“filled”) cell are intersecting. You may have not ben aware of having used cells in that range.

If you suffer from unexpexted behaviour of the software in general, please supply info about:
OS / version, last upgrade;
LibO version, when istalled;
examples of what you did exactly and what reaction you got exactly specified for each mentioned case.
Otherwise it is not very likely you get a helpful answer.