"Formula custom fields not allowed in expression"

"Formula custom fields not allowed in expression"

I just starting creating reports that I had setup in Sales Force within Zoho. The reports use custom fields which I have created in Zoho and I am using the FORMULA customer field creator. However, none of my formulas are working and I am getting the following error when checking for any SYNTAX errors: 

"formula custom fields not allowed in expression"

I am not familiar with this error and Zoho doesn't provide any specifics to the error. 

Even the most basic formulas are not working properly. For example: 

(${Potentials.BIS Expected Revenue for Software}+
${Potentials.Hardware Amount for Gross Profit})

This should simply add these two fields and not give me any errors. Please explain?