Turn on the underfloor heating at the controller

The actuators are missing here. Or the heating installer is knowledgeable and only performed the thermal balancing via the valves.

I wouldn’t use any if the upper floor is now to be heated permanently, but instead set them manually. Of course, this only works if the heating curve fits; otherwise, you have to lower that as well until it fits.
The blue caps are actually "construction caps" or manual controls. The further you turn them in, the less flow there is. Fully up or "off" means fully open.
I would first open them all and then see how everything behaves. Especially whether enough reaches all heating circuits on the ground floor and if there is enough flow everywhere. If anything doesn’t fit: turn down where there is too much and wait another day, etc.
If necessary, do the same for the heating circuit distributor on the ground floor if the valves there are throttled too much.

I can report that I have opened the cold and hot water pipes. Then there was a lot of water noise and gurgling, and now everything has stopped working. The error message reads F 532: Flow rate too low. It is a Vaillant heat pump. I don't know exactly which model. The installer is coming on Thursday or Friday. Such a mess. I didn't know that turning on the heating is so difficult.

And just refill and see if anything is leaking?

But this installation already looks adventurous. Why is nothing insulated, actually? Is the person who installed it back then coming?

Then there was a lot of water noise and gurgling and now everything has shut down. The error message reads F 532: Flow rate too low.

I don’t know the hydraulics or the circuit diagram, but probably it just “choked.” After three years of inactivity, there was probably no pressure in the system, but air here and there. Now the water pushed into the circuit, flushing out all the air, which is why there was gurgling. Then there were too large pressure fluctuations or too much air at the heating pump, causing it to go into fault mode. If there is still enough pressure in the system (usually more than 1 bar), I would simply acknowledge the fault and restart the heating. If the flow rate is still too low, it will shut down again because something is faulty.

The pipes I opened were unfortunately the pipes to the radiator in the bathroom. However, there is no radiator installed there, so the water ran onto the floor. Wooden floor as well, what a mess. Now the error message about insufficient flow rate also makes sense. I really hope the installer will fix everything.

I can report that I opened the cold and hot water pipes. Then there was a lot of water noise and gurgling and now everything has stopped working. The error message is F 532: flow rate too low. It is a Vaillant heat pump. I don't know exactly which model. The installer is coming Thursday or Friday. What a mess. I didn't know that turning on the heating is so difficult.

I know that "problem" well from the Panas. It regularly occurs there during commissioning. 1. Increase the pressure in the heating circuit significantly. Preferably above 2 bar. 2. Bleed wherever bleeding is possible (Pana with bleeding program, also bleed at the heat pump) 3. Start and be happy when it gets warm Basically what also says.