My brother can't login in to Minecraft Java after it Java and Bedrock became one

Sadly it doesnt work and i cant try the window 1/8 launcher trick because i dont have enough memory

Thanks for the help i will try it out

Could you try uninstalling the actual minecraft launcher and install the Windows 7/8 Launcher momentarily?

Try following steps from the official bug report (take a look at the comments)
https://bugs.mojang.com/browse/MCL-20496

If the issue persists, Mojang advised to submit a ticket with them Idriss

Cheers

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today Idris

Do you mean that he's receiving a message that says something like "failed to create a profile"?

If yes, actually this is with the latest minecraft launcher versions;
https://bugs.mojang.com/browse/MCL-20496

I'm going to share you something that worked for me:

This is (if you're able to play Java without no issues) sign with his account on your PC and Java launcher, once there try creating for him the username and that's it

I found this workaround due to I was facing the same issue on my Mac (and I never played Java edition before), I asked my brother and... Bum
I was able to play it

Also your brother could try downloading the launcher for Windows 7/8 from here and give it a shot:
https://minecraft.net/download

My brother got the java +bedrock bundle but he simply did not get java edition.