I’ve found the solution. The LemmatizerTrainerME is inside opennlp tools jar file. So that’s what I did:
I ran Windows Powershell inside lib folder with the following command: opennlp opennlp-tools-1.9.3.jar LemmatizerTrainerME -model en-lemmatizer.bin -lang en -data /path/to/en-lemmatizer.dict -encoding UTF-8 and it worked.
TLDR: I ran Powershell inside the folder that contains opennlp tools and added the tools file name before the arguments so it could access LemmatizerTrainerME
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