Error asking the user to re-enter their nickname in Java

short description of my situation

I have bought the game since November of last year. When I bought it, I went directly to the minecraft.net page and reserved my Nick and put my skin to play in the Java version. Since I didn't have a PC back then, I used to play the Bedrock version. Now that I have a PC to play on, it asks me to re-enter my nickname in the launcher. I do, and it tells me it's not available. I check those pages where you can see if a nickname is available and it says that if it is available, I don't know what's going on.