You can find the STP Diagnostics at Diagnostics → STP.
Here you can see all Spanning Tree Protocol information. It’s divided into Bridge, Tree, Port, MSTIs, MST Conf. ID, VID-to-FID and FID-to-MSTID. You can select the desired bridge to get the information.
To revisit your bridges STP settings please go to Interfaces → Assign → Bridges and refer to the Bridge documentation at Bridge.