Cell reference is formula, but getting #Value error?

The values in each of the Points columns are all empty string "". Excel does not know how to add strings together (using the + operator). When you try to add text strings together like this, Excel throws an error.

Solutions depend on exactly what you want to do.
You could use SUM() instead of +. =SUM('Billing Summary'!E2,'Billing Summary'!E15). The SUM() function knows how to ignore text and would return 0 instead of an error.
You could have the Points formula on Billing Summary return 0 instead of empty string "".
or something else that fits into your project better.

Help us understand how you want this to work when the points formula is returning empty string and we'll see what we can suggest.