The antibody code chops the FASTA sequence up into subsections, for the various constant regions and the CDR loops. One of those regions is abbreviated “FRL” – probably framing region light, the not-CDRs in the light chain.
The database (DB) only has 58 and 60 residue options for that region. You’ve submitted a FASTA that it thinks has 59 residues in that region.
I guess the easy solution is to try deleting or adding a residue someplace innocous (the termini?) to see if you can get it to 58 or 60 so it will run?