Sign in Error on POS because of Azure AD account is not mapped in D365 Commerce (Retail)
Many customers use Azure Active Directory (Azure AD) to manage user credentials for those services. In those cases, the customers might want to use the same Azure AD account across applications.
Before a store worker can use an Azure AD account to sign in to the POS application, the Azure AD account must be associated with that worker.
At times you will get an error while sign in POS for employees. Even when you add Associate Existing identity for the worker in HQ.

Follow the below steps:
Go to Azure portal, Click on Azure active directory.

Go to user and select the user


Copy the External Sub Identifier.
Inside AX HQ, go to Commerce shared parameter
Inside Identity Provider, Add azure AD link.
Inside relying parties Add user External sub identifier for the same.

Inside https://commerce.dynamics.com/auth provide below relying parties.

Inside https://commerce.dynamics.com/authcc provide below relying parties.

Run 9999 job from Channel database. After full Sync Activate POS again. you will able to sign in successfully.
hope this helps!