Make sure you enable this option when you create your apps.
Is your app connected to a licensed node? That would explain why the “Publish” button is disabled.
Another route to your sandbox environment is: (1) go to the Developer Portal, (2) open your app, (3) go to “General settings” > “Access Management”, (4) click on the link in the “Sandbox URL” card. You can deploy your recent changes through “Changes” > “Commit”, but it seems that you’ve already done that.