This error means that you have sample IDs in the OTU table which are not in the metadata. You should double-check that the sample IDs in the biom file (or, if biom format confuses you like it does me, the column labels in the .txt OTU table) are found in the #SampleID column in the metadata file.
It looks like this is probably the problem: I'm guessing that your ps_r_otu.txt file has no column names, given that you passed in col.names=NA.