Hi Phalguna,
If you create Mendix app in 10.12 version & try to open in 10.15 then it will work because version upgrade is easy from lower version to upper,
But if you do vice versa upper to lower it is difficult.
Try this
https://vstram.medium.com/is-it-possible-to-downgrade-a-mendix-project-file-c1b194382c1c