unlock icloud by invoice

hello. i have a problem with my iphone X . i bought my iphone X in 2019 and I gave it to my cousin so that he should use it and used it for 2 year but some few month ago he left us . but in the phone is he icloud account . i tried to restore the phone so now the phone is on the icloud activation and i dont know his icloud account info . so please how can I unlock my iphone by

my invoice ??