How to Format SD Card to Internal Storage with Nokia 6.1?

My brandnew Nokia 6.1 has used more than 13GB just for system files, so I want to move some other apps such as games, photos… to an SD card. I have found the instruction on how to format sd card to internal storage online but can't apply it because there's no "format as internal storage" option.

Could anyone who has done it successfully please give me the tip? Thanks a lot in advance! Cheers