Shimano caliper not sitting properly on fork (gets angled)
First double check the wheel is fully seated in the frame. If it is, and the caliper is sitting at an angle no matter what you do, it's a sign the post mounts need  to be faced, which essentially is a shop job unless you buy a fairly expensive tool. This isn't common on new suspension forks for whatever reason, although you see it out of the box on frames quite a bit presently. Hopefully whoever you bought it from just takes care of it for you.

A related problem arises if the mounting bolts have been overtightened and caused the post mount or adapter to dent a depression into the surface of the caliper. That doesn't really happen on relatively chunky mounts like these though, more the dainty rounded ones that some bikes use.