Add Cells, but ignore those when text appears

[Quagga]

Some data from tables have the word "INVALID" in them because this option is not available.
This word comes up in my formula and gives me #VALUE.
How do I get it to ignore the text "INVALID" and can I be warned if I enter a number in a cell that is linked to this "INVALID" text.

I have attached a Test file to try to explain what I want to achieve.
Hope it all makes sense to you.

Thanks All