Cannot change the summary detail in the values section of a pivot table

Hi all,

I'm hoping someone can help me out. I've created a pivot table using numerous tabs and the data model. All was going great until I tried to change the summarise part in values from count to sum. I get an error message saying it cannot do this as it's not a supported calculation for text data types.

There is text data in my information but the info I'm adding to the values field specifically is numbers only. I've checked the formatting and made sure it's all numerical. Any ideas how to resolve this?

Really would appreciate any help!

Thanks