Wheels Question

Hello guys. My wife drives a 21er Grandland with 1.5l D. 131 HP engine. Now I wanted to order a complete bike with summer tires, as such were not included in the purchase. Had me at the Alufelgenshop.at for Dezent TR silver, in the size 7.5×18 with the ET 48 decided. Then got an answer that they need the vehicle license to check my order. After sending them they said that these rims with my brake do not fit and whether I switch to other rims. On my question how this can be, since original 17″ are installed, they meant that it does not fit with the ET. Now my question whether someone has already had experiences in this direction. Can’t imagine that the ET has an influence on this. Especially since these rims can be selected for this car in all rim shops. Thank you already in advance.