Try this, Once the Main Form Opened,
1. Select/Click on the SECTION where the field is Available.
2. Click Change Properties at the TOP.
3. Click Formatting and Navigate to Field Label position and Select TOP and Click ok --> Save --> Publish
4. Next Switch to Classic mode and publish from there and close the window.
5. and then publish again from make.powerapps.com model driven app editor.
Here you go.



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