Assuming you meant multi-user.target I was able to reproduce the issue you describe.
Interestingly it doesn’t seem to be related to confinement, as I installed the snap in devmode and got the same issue.
It’s not related to the systemd boot target, though. Booting into graphical.target, switching to the text console and logging in there and starting a vnc server ends with the same error.
It’s not just the snap-store snap, either: installing xbill-xaw you also get a “cannot open display”.
Something for @zyga-snapd to look into in the morning, perhaps.