One user experiencing access issue

Hi all, 

We have one user in our organisation who is unable to access our SharePoint page. 

This user is granted access through an outlook group. Every other person on the list has no problem with accessing.

I've tried manually adding access outside of the mailing list, and also removing and readding access, and the same error persists over all attempts. 

I have attached an image of the error below. 

Edit: Same error presents on PC and mobile. 

Appreciate your assistance :).