You may enter incorrect values for Location and/or /codon_start of CDS feature.
If the value of /codon_start is either of "2" or "3", the location of CDS feature should be 5' end not complete.

See Description of Location and modify the location with flag for "5' end not complete", for an example, from "1..300" to "<1..300".
When the CDS feature is started with an initiation codon, correct /codon_start with "1".

Offset of the frame at translation initiation by codon_start
How to fix error message: "First codon [***] is not a start codon." / "Final codon [***] is not a stop codon."?
How to fix error message: "Stop codon ‘*’ is found in the range."?