If you use the free version, please set https://api.languagetool.org/v2 as the base URL and leave “Username” And “API key” empty.
If you’re a premium user, use https://api.languagetoolplus.com/v2 as the base URL and set your email as the username and the API key from here).
With a local server, use http://localhost:8081/v2.
You might also need to activate other settings. Here’s a summary of those three settings:
You might need to close and re-open the Options dialog to make sure the next setting becomes available:
