I need help, I am not a pro... (and sorry for my english! )

I have a Cube Nuroad C:62 Pro with a Shimano GRX RD-812 and one cassette Shimano SLX CS-M7000. Is there a trick to mount the cassette on the Elite Suito? I had to install one spacer even if it is a 11 speed cassette. Now, there is noise on the transmission and shifting gear is not really smooth... Any clue? Thanking you in advance!
Regards,
Michaël