Issue
Not able to select the created User to be assigned to an Account.
Environment
Liferay DXP 7.4
Commerce 4.0
Resolution
Users for Accounts can only be created within the Account Users interface. A regular DXP User that is created in the Users and Organizations interface cannot be associated with any Accounts. However, an Account User can be managed and edited by Users and Organizations after creation.
The current functionality is documented and intended to work like this:
https://learn.liferay.com/dxp/latest/en/users-and-permissions/accounts/account-users.html
So the official recommendation is to use the Applications → Account Users, to create the User.
For the time being, the proposed way to assign a regular User to an Account, is through an Organization, as you can assign Users to an Organization, and you can assign that Organization to an Account.
Additional Information
Story for this behaviour in order to be able to add regular Users to Accounts: LPS-149125.