@Paul Snedden - While you wait for an answer here, have you opened a help desk ticket?

@Max Harper - That app ID looks familiar: https://miro.com/marketplace/clusterizer/?backUrl=%2Fmarketplace%2F

Any ideas?

@Paul Snedden - Opening a ticket:

Thanks for the tip, Robert. I’ll raise the Helpdesk ticket now.

Just in case, I’ll duplicate the Support team’s reply here:

Since you've limited the application installation to a list of the approved apps, before installing any new application you need to approve it. As a Company Admin you must have permissions to do that. 
Please go to Account settings > Approved apps, fill out the app ID (3074457350598358773) in the field and click Approve:

Then try to install the app again. It should work

Perfect, thank you.