STP is not enabled as it is not required in a loop-free topology.
Any of the supported spanning tree configuration modes (STP, RSTP, MSTP, PVST+, R-PVST+)
Enter no shutdown to re-enable any of the supported versions of STP.
To disable RSTP globally:
device# configure terminal device(config)# protocol spanning-tree rstp device(conf-rstp)# shutdown
To enable MSTP globally:
device# configure terminal device(config)# protocol spanning-tree mstp device(conf-mstp)# no shutdown