@Vishnu Anand, Thanks for posting in Q&A.
For your issue, I have done some research, here are some information you can refer to.
Based on the official document, it says much of app protection functionality is built into the Company Portal app. Device enrollment isn't required even though the Company Portal app is always required. For Mobile Application Management (MAM), the end user just needs to have the Company Portal app installed on the device.
When using APP, the company portal app simply needs to be installed on Android devices and shouldn't be signed in to. If you try and sign in to it, it will try to enroll the device, which you don't want and will get the error message.
So, for unmanaged devices, there is no need to login Company portal to get the app protection policy, we just install it and then the app protection policy can apply automatically.
Hope above information can help you.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.