One of our customers was having issues with changing the site logo for a Modern SharePoint Team Site. I found this strange because he was the owner of the Team Site and I was able to change the logo. What was the issue? Well! The Modern SharePoint Team Site is of course connected to an Office 365 Group. The customer was only owner of the Team Site and not the Group! After adding the customer as Group Owner, he was able to change the site logo. Hope this helps other people out there with the same issue.