Can't enable System UI Tuner
It seems that the System Tuner UI was removed starting with LineageOS 14.1, see this reddit post. The settings which were to be found there are claimed to be integrated with the standard settings – and from the screenshot of the UI Tuner I've seen, this indeed seems to be the case (My BQ is also running LOS 14.1, and I can't enable the UI Tuner either).