Error with calculation command

Hi @Afolayan
When you get a chance, please introduce yourself here. I'd also encourage you to add a real picture as your avatar because it helps build community!

You are trying to refer to a question that is in a repeat group. To do that you have to use the indexed-repeat() function to specify which value you want (from which group) as the error message says.
You can read about that function in our docs https://docs.getodk.org/form-repeats/#referencing-repeated-questions-from-outside-the-repeat and also find many sample forms on this forum (just use the search icon and enter indexed-repeat).