Perhaps this forum post might help?
Your SharePoint list fields type should be the same type as used in PowerApps fields type. i.e. if using the dropdown in PowerApps, you need to make sure you are using Choice in SharePoint list for the same field.
I doubt this is the problem, @Muzammil .
I've encountered this error numerous times and the only solution so far is throw away the fields and re-create them. It's a back end problem that should be fixed.
Also have run into this issue today - was working on a Training Form linked to a SharePoint List. Was working perfectly fine yesterday, now suddenly 'data source may be invalid'. Hovering over the submitform button, it just says 'an unknown error occurred'.
I went back to the default form and it creates a new item with the list fine which means there's definitely something not working with the PowerApps submitform. Extremely frustrating.
I found what was causing my issue. The data was invalid.
Sounds like a stupid reseponse, but I went back through the data, found a required column that I wasn't looking for, thats one issue, but didn't solve it.
However, I found a tile in the form that represented a lookup column. This grabbed the Project Number when you selected your Project Name. I thought this would auto fill in the form as you select the name, but it didn't.
Once I deleted this tile (that wasn't doing anything anyway) I was able to create new items again.