I think it’s due to your network issue, right now i can able to access appstore successfully. You can track the Mendix Maintenance and cross check
If you still have this issue, please register a ticket with Mendix Support.
For everyone who reads this that doesn’t know how to do that: On the top right of this page there should be a circle with a ‘?’ symbol in it. Click on that symbol and a drop-down menu should appear. One of the options there should be ‘Contact Mendix Support’.
After clicking on that menu option you should arrive at a new page which in the top right corner should have ‘Submit a request’. Fill out the form and submit.