Windows Excel protected sheet opened in Numbers
Numbers is not Excel.

Numbers can only do a subset of things Excel can do with Excel spreadsheets.

If you have done not basic things in Excel (eg protection) you need Excel to undo those things.

This would be true if the original was Numbers and you used Excel on Numbers file.

Each application is different and an export of their specialized file format loses some functionality.

But then again there are things that only Excel can do e.g. Pivot tables.