Hi Steve,
is it a sandbox/free app or do you have a mendix license with a cloud node?
If it is a sandbox/free app, it probably has something to do with the current problems mendix has with the freetier (see: https://status.mendix.com/incidents/fkld33drkhzg)
For licensed apps, the "run in the cloud"-button is deactivated and you will have to commit your settings to the teamserver and build a deployment package in the environment portal.
Hope this helps.
Best regards,
Alexander Keßler