STO is not a fault code.

It is a state of the drive.  It means that Safe Torque Off function is active. The drive is not detecting 24Vdc on the STOA or STOB terminals.
Measure for 24Vdc between +24 to COM to verify the internal power supply.
Confirm that the factory installed jumper between +24 to STOA & STOB is solidly connected.  The jumper is removed if a safety circuit is installed.  Verify continuity of the safety circuit (if installed).
Make sure the Sink/Source switch is in the Source position.
When the voltage on STOA and STOB is re-established, the drive status will return to RDY if there is not an active run command, or NST if a run command is active.  To clear NST, remove the run command.

If STOA and STOB terminals are at different voltage levels (one on and the other off) a SAFF error will be triggered.

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