If you preview the report in Crystal Reports when the error appears, the formula will open.

Summary field: Sum({Field Name}, {Group Name})

Verify that the group field of the summary is valid. For example, if the summary is Sum({CnGf_1_Amount}, {CnBio_SortKey}) the group field is CnBio_SortKey. Should there be a group on CnBio_SortKey? Are you trying to sum the Gift Amount field for each constituent? If go, make sure there is still a group on the CnBio_SortKey. If not, change the group field in the summary.