Hi Naveen Kumar Somanagoudar ,
This sounds like user do not have the privilege to log in ERP. Appreciate if you can share any screen shot for this.
You have to check the following.
--- Please ensure that the user ID of this approver is set to Enabled in "Users" form in system admin module.
--- Also make sure that "Person" is linked with this user ID in 'Users' form.
In addition to that; please make sure below are configured properly in terms of avoiding any further issue related to it.
--- Appropriate roles have been assigned to this user.
--- Check if proper privilege has been assigned to this user to approve the PR. You can check if any such privilege is available in AX "Approve purchase requisitions". Below screen shot is taken from D365 for your understanding.

--- If your workflow configuration is designed based on participant or even hierarchy based; then make sure that "Position hierarchy" is assigned properly as well as a department is assigned to this user.
Kindly review and update in this forum.
Best regards,
Sourav Dam
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