Excel has a hard limit: 1,048,576 rows and 16,384 columns (XFD). If there's any data (even a space, formatting, or formula) in the very last row/column, Excel blocks all inserts.

Here are a few things to check:

1. Scroll Lock – The most likely cause. Check the Excel status bar; if it says Scroll Lock, press the ScrLk key (or use the on-screen keyboard if needed).
2. Extend Selection mode – Press F8 to turn it off if the status bar shows "Extend Selection".
3. Pending cell edit – Press Enter or Esc to complete or cancel any edit in progress.
4. Stuck keys – Tap Shift, Ctrl, and Alt a few times to unstick them.
5. Protected sheet – Under the Review tab, see if the sheet or workbook is protected.
6. Restart in Safe Mode – Close Excel, press Windows + R, type excel /safe, and test. If it works, a misbehaving add-in may be the culprit.

Give those a try, starting with Scroll Lock—it's usually the sneaky one! 

My answers are voluntary and without guarantee!

Hope this will help you.