Hi @Aigars,
Some guidance below for visitors to the topic.
There was a recent update and error underlining was not working until the fix in Processing 4.4.7:
Be careful not to delete the sketches you created!
Create a backup if you choose to delete these.
User sketches are here:
I also had some error underlining issues too with VS Code and Processing 4.4.7 but it was related to all my tinkering with VS Code settings! Entirely my fault in this case.
I did a VS Code restore for this and everything worked after:
I am careful now and document what works and able to restore easily to defaults.
VS Code is very much a learning curve for me as well!
And still learning things about the Processing IDE.
I have multiple versions of Processing and it sometimes gets complicated.
I have since learned to isolate each environment so they do not impact each other.
:)