Are you a professional fiduciary?
If you are not a professional fiduciary, then you do not need a business code; the fee you receive should not be reported on Schedule C (it is miscellaneous income).
If you are a professional fiduciary, you can always use "999999" for unclassified or choose a code for one of the professional services.
See https://www.irs.gov/instructions/i1040sc#idm140094050353104
