Yes, please add an instruction only to use postgres to the description, or switch off SQLite, as the error is still there.

Seems to be good idea as many issues just disappear...

Yes, you do have to run the test suite once you have written a function that completes the required functionality. There is a button in the bottom right hand of your editor (by default). You can change the view of the editor so it is possible it is in another location. You can search (ctrl+f on windows) for "run suite". That will execute the suite of tests for a particular kata. Once all the available tests have passed it will let you refactor the code before final submission. The final submission button will be in the same location as the 'run suite'.

I hope this helps. Good Luck!

maybe I should add that to the description, all my katas should be completed using POSTgreSQL.

I think there might be issues with the final suite test. I finally had to unlock the solution because I couldn't figure out the issue. I then looked at accepted solutions and when I tried on of them it had the same error.
Is the kata still working as expected?

I was using SQLite 3.2.8. Strangely the solution from SteffenVogel_79 appeared to work fine when using POSTgreSQL. But I didn't think I was using something specific to SQLite.