No contacts available - no landing request possible

I was travelling with several hops in order to reach the final destination and in each system I stopped in during my travel I scanned the system in order to collect data (for the first time I was using my scanner).

But then it turned out that I was not able any more to request landing on any station. The stations were targeted, I was not in supercruise, I was located 1000-200 metres away from the stations respectively and my weapons have been retracted, but in the "Contacts" tab nothing was listed, so I was not able to request landing.

Somewhere I've read I'd have to enable my radar, but I don't know how to do that. Was is my radar? Or better: How can one turn on/off the radar? I didn't find anything like that in the command options.

What's going on there?