Not able to calculate in 'Prepare a Form' tool

This is a summary of my issue: 'Prepare a form' tool in Adobe Pro - the 'calculate' tab won't show up in 'field properties' in a given field, I am trying to do some basic SUM calculations - but the 'calculate' tab is hiding somewhere!

I know the 'calculate' tab (along with some other associated tabs) must be locked at some level - but i cannot tell which or where.

If Adobe Acrobat Standard users are familiar with this type of problem and are able to point to possible clues or direction , i will be most grateful.

Thanks,