When using trim.seqs with an oligo file containing only a reverse primer, I get a segmentation fault (v. 1.25.1, linux executable). However, there is no problem when the oligo file contains only a forward primer or both a forward and reverse primer.
You likely don’t want to provide the reverse primer - I suspect if you look at the trim.fasta file that is created, it’s probably empty. When you provide the reverse primer, trim.seqs expects the sequence to end with the reverse primer. If you sequenced from the 3’ end of the gene back towards the 5’, then you should rename the reverse primer as forward and use flip=T to get its reverse compliment.