Hey @Aaron_V,
I was recently testing a few solution ideas through SM and from my experience, the 'Add a new device' button is related to Trusted Access enrolment and not Management Profile enrolment.
https://documentation.meraki.com/zGeneral_Administration/Cross-Platform_Content/Trusted_Access_for_S...
Trusted Access is an easy, secure way to connect iOS, iPadOS, macOS, and Android devices to Meraki MR wireless networks without enrolling the device into Systems Manager.
Adding a device for management purposes is still via the options presented in the Add Devices page on Dashboard. Email, mobile App, Web brower..etc