Hi @Sumaira ,
According to the error message that you mentioned, I think this issue is related to permission. Have the user been assigned with proper Web Role with sufficient permission to Document Location Entity in your current CDS Environemnt?
Do you add corresponding Contact record for the user in your Portal Management?
Further, does the user have enough permission to access the Account Entity in your CDS?
Firstly, please make sure the user has sufficient permission to access Account Entity in your Portal Management. And has been assigned with proper Web Role with sufficient permission to Document Location Entity.
According to the issue that you mentioned, I have made a test on my side, and and don't have the issue. The screenshot as below:

The "Authenticated Users" web role has been assigned with the following Entity Permissions:


Please make sure you have assigned proper "Web Role" with sufficient permission to the user in your Portal Management as above.
More details about assigning Entity Permission when integrating PowerApps Portal with SharePoint, please check the following blog:
https://www.inogic.com/blog/2018/11/sharepoint-integration-with-microsoft-portal/
Best regards,