Recently when I was working on a Model Driven app, I had observed that the App’s Play button was disabled in the Power Apps Maker Portal.
Later, I realized that I had added some features to the App and Saved the Model Driven App, but not Published.
Clicked on Publish to Publish the App.
Then, I was able to see the My App’s Play button in the Power Apps Maker Portal.
Hope you have followed all the steps and resolved this issue.
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My earlier articles related to Dynamics 365 CE that you can have a look.