Ctrl&End will bring you to the last used cell in an Excel spreadsheet.
It sounds as if you are using close to the limit of 65,536 rows and if you insert the 'rows' Excel would have to drop off some rows at the bottom of the sheet. (Excel in its wisdom is alerting you to the fact and refusing to do it!)
I guess that you don't have 65,536 rows of useful data but you have probably, in error, filled some column right down to row 65,536.
Ctrl&End to see what is in the last cell/used row of your spreadsheet. If it is data that you have put there then you can't insert rows above without losing data from below... If it is entries you have put there in error then delete them and then insert away...