Radek

If you have ruled out all problems and the possibility remains that there are faulty pins, then it is best to solve the problem with the seller as a guarantee....

There can be several problems. - short circuit of the heated bed to the frame - poorly connected bed - defective MOSFET switching transistor - defective pin on the processor...

Just for info. The positive voltage is permanently connected, the negative potential is switched

Did you short the hotbed? Are you sure you know what you're doing? This is how you must burn a Mosfet. Did you use Marlin for Ender? Was this FW...

How you used Merlin. Do you have the configuration files you used?

Do you have a Volt and Ohm meter?

Check Mosfet: Unplug the printer. Disconnect the bed heater from the SKR Perform the ohm measurement as shown.  How much did you measure?

If you had unplugged the bed and measured these values, it means that you have burned mosfet. There are two solutions. Solder a new mosfet, or use an external mosfet....

Yes, if the board is under warranty, repairing the board will void the warranty. If the board is under warranty, contact the seller and request a warranty replacement.

Close the ticket if you no longer need it.