Hi @rafaeldamasio,
Generally the cause for this particular behavior is that your cell is formatted as Text.
To fix this error, just select the cell, set its formatting to “General”.
When a cell is formatted as Text, Excel makes no attempt to interpret the contents as a formula.
I had this exact same problem some time ago, and i found the answer here:
If the problem continue, you can find some other reasons there.
Hope that works.