Hello Lauren,
This KeyError during vsearch clustering seems to happen when reads are filtered out because they are too short.
And in your error message, I also see mention of a read being removed due to length:
Reading file /tmp/tmpuq4x0ruu 100%
440539 nt in 2008 seqs, min 39, max 310, avg 219
minseqlength 32: 1 sequence discarded.
That could be the ASV labeled db16c939e7d01eaf1a3554820a848d44 which is missing later leading to that key error.
This great post by ebolyen has more details about this error. Can you double check on this ASV and see if it is < 32 bp long to verify that this is the source of the problem? Thanks!
Colin