On sandbox, it seems like on the "Associate Dev Kit" screen, the only merchants that we are able to associate with a device are merchants that are still in the "Setup Pending" step.
The only way to get to this step is to do the following:
1. Create a new merchant/test location
2. Navigate to the merchant/test location's dashboard
3. AVOID clicking the "Is this location part of an existing business? Yes No" modal at the top of the screen.
I currently see no retroactive way to get an existing merchant into the "Setup Pending" state. As a result, I am not able to re-associate my devkit with a pre-existing merchant. Only new merchants that I create and leave in this "Setup Pending" state are able to be associated with a device.
This seems like a bug to me. A device should always be able to be associated with a merchant, regardless if they are a fresh new merchant that is pending setup.