Follow the password rules for TMSADM (SAP Notes 1515926, 1414156)

What causes this issue?

You are attempting to create a domain link. There are currently
restrictions with respect to the TMSADM password.

System Response

Note for the administrator

How to fix this error?

The administration of the TMS confiuration is simpler if the TMSADM
password remains the same across the domains.
Caution.
Older releases do not implement a flexible standard password. Domain
links to older system landscapes are either not possible or you can
choose one of the two recognized standard passwords. See also SAP Notes
1414256 and 761637.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Procedure for System Administrators

A standard password carries the risk of unauthorized parties viewing the
content of the transport directory.

Error message extract from SAP system. Copyright SAP SE.