Why did Serbia give up comparison letters from Kosovo: Catalonia

The reason for this, in novost, shows Serbian President Aleksandar Vuciq:

I learned that the letter was not sent to the EU. In spite of that, we would send the letter if Spain did not directly address us in prayer not to do so because we would make matters worse. We gave up and that's fair, because Spain was fair to us. Many times. I know all the pressures Prime Minister Rahoy has had him release to recognise Kosovo, and he has recognised Kosovo. I also talked to Spanish King Phelipe”.

In this letter Serbia had raised the question of why Kosovo makes independence declare it cannot do Catalonia.