You must convert the field into number, and then try to get the sum.
Step 1: Create a new formula from field explorer, give a name to formula (relevant name).
Step 2: Convert the field into Number.
Step 3: Then Add Sum of the formula field
Step 4: Place that Formula field into the Report Footer.
Add Formula:
Choose Field to Convert into Number or Currency:
Then After Field Conversion, get the Sum of the Formula field by choosing name of the formula from the Report fields. Formula field has fx as prefix.