When you try to input some value in a cell and you get an error message in Excel 2016:
or in Excel 2013:
This means that someone has set Data Validation on that cell. To view its configuration and remove if needed follow these steps:
Select cell and go to Data -> Data Validation
Depending on the size of the window you access it in a bit different way.
Please look at the screenshots below:
You will see Data Validation window. Look into Settings tab, Validation criteria section.
If you want to disable validation, select Any value in Allow drop down and accept by clicking OK.