Please enter following formulas and let us know if it gives error -
= 1 + 1
= A1+B1
=SUM(1,2,3)
=SUM(1;2;3)
It it gives error in all cases, I will suggest that you repair your Excel.
Repair the office installation - https://support.office.com/en-NZ/Article/Repair-an-Office-application-7821d4b6-7c1d-4205-aa0e-a6b40c5bb88b