This Action is not allowed as Transporter is already assigned.

Self-Explanatory Message

SAP has defined this message as ‘self-explanatory’ and therefore, has not provided any further details for it.

All messages in SAP have a message text (shown above). However, the message text is not always useful enough to understand or resolve the issue.

Therefore, most messages in the SAP system provide additional long text with details about what caused the issue, how it can be resolved, what actions to take or configuration changes to make, etc

Unfortunately, this specific error message does not provide such additional information.

What else can you do?

Even though this error message does not provide any useful or additional message details, you can still try to solve the issue by searching on SAP’s support portal. In many cases, there could be an SAP support note that provides further error explanations or even directions for resolving the error.