If you are getting an
Invalid plan or unable to access error message while trying to transfer devices from one organization to another it likely means that there is an edition mismatch between the organizations or, if you have one, your custom plan is not available on one of the organizations.
To solve this issue you simply need to make sure that the originating organization and the receiving organization are on the same edition and that that edition matches the SIM's plan. So, for example, if the SIM is on the Professional Flexible Plan both organizations must also be on the Professional Flexible Plan. For new organizations you need to go through the first step of the Activate your first SIM flow to select your edition.
Here is the guide on upgrading your organization's edition.
If you are running into this error due to a custom plan please reach out to your sales representative who will help you resolve the issue.