When you first looked into this problem, did you see this fantastic post addressing the same error message?
In short, " that error is only raised if your first line (ignoring any blank lines) does not start with a >", and certain software may introduce “invisible” characters that precede that > in your fasta. @Oddant1 describes a diagnostic approach (hex editor) and the OP describes a straightforward solution (delete and replace the >) if you find your issue is the same.
Let me know how it goes!
CK