I got the following output when I used remove.otus:
Linux version
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[NOTE]: Setting random seed to 19760620.
Batch Mode
mothur > remove.otus(accnos=List_OTU_2b_removed_small.txt,constaxonomy=final.taxonomy_TEST)
[ERROR]: 00783122022122471063711866RAM used: 0.00453949Gigabytes . Total Ram: 754.39Gigabytes.
has occurred in the Utils class function mothurConvert-int. Please contact Pat Schloss at mothur.bugs@gmail.com, and be sure to include the mothur.logFile with your inquiry
It looks like your txt file has the names of OTUs you want to remove, but your taxonomy file has sequence names. The taxonomy you want should have originally had a name that ended in something like cons.taxonomy.