When adding words to the userdict.tlx lexicon file to support the spell check feature via the pdm_lexutil command, we get the message "The lexicon to be modified cannot be located in the lexicon list. Some errors were encountered processing this request."
Copy the file userdict.tlx from Service Desk\lexicons ($NX_ROOT/lexicons) to Service Desk\Site\Mods\lexicons. Then, recycle CA Service Desk Manager services.
Now when you run "pdm_lexutil -a" or "pdm_lexutil -a -f inputFIle" to add words, the message should not occur anymore.
Note: Since it requires restarting CA Service Desk Manager services, perform it after hours if it is on a production system.