I bought the Windows 10 retail key and it worked perfectly! It is really retail, not the OEM. And it activated on my 21H1 build.

Perfetto arrivato in un secondo e funzionante

all goood :) keep up the good work

Received instantly, it works perfectly, 100% recommend.

The key didn't work at first time
If you have the same problem i had, dont panic
Go in support and ask for a refund or a new code (they give you the code in the private conversation) they are reactive but it can take more than 15 minute to have a new working key

They need:
Order ID
W10 key who don't work
i also add a picture that proof it didn't work