Is the formula = ((A2+B5)*5% valid?

No, the formula is not valid. There are instances when excel will see the formula that you have placed in the cell as text. This means that there is something wrong with your formula because it is not considered to be one of the formulas that excel can use in order to compute.

There are times when the formula is correct but because of the text format, it is considered to be wrong. When you get data from your CSV file or notepad, this also has a higher chance of happening. You can always double check the formula to see if you have written something that should not be there. Once you input the right data, you will also get the expected results.