I realize that this is an old thread but thought I would update with information that would be relevant in today's environment.  I had similar issue and the solution was to enable VLAN 1 on the core switch and walk it down to the other switches.  After doing so the switch status will update to point to the Core Switch as being the Root Switch in the topology.  Here is an article that was provided to me for further reading.   Identifying Root Switch:  https://documentation.meraki.com/MS/Port_and_VLAN_Configuration/Determining_the_RSTP%2F%2F%2F%2FSTP_Root_Bridge_on_an_MS_Switch_network   Configuring STP on Meraki Switch: https://documentation.meraki.com/MS/Port_and_VLAN_Configuration/Configuring_Spanning_Tree_on_Meraki_Switches_(MS)   Advance Setting of STP on Meraki Switches: https://documentation.meraki.com/MS/Deployment_Guides/Advanced_MS_Setup_Guide#Spanning_Tree_(STP.2C_RSTP) ... View more