I have a small problem when writing the formula:
When you finish typing the formula, press "Enter" but not the result, but the formula is always there.
For example: "= Data! A4"
The result is "= Data! A4" but it does not count.
Firstly, there should be no quotation marks surrounding the formula. Second, the space between the sheet name and the cell address should not be there.
If these are not the issues, then other possibilities are on the ribbon, you have selected: Formulas-->Formula Auditing-->Show Formulas.