The spark electrode is used to provide ignition BUT is also used as return path for a milliamp flame proving signal..
No return of signal and circuit board shuts down gas valve...3x lock out
Spark electrode \
Could be dirty/sooted up....clean with emery cloth
Out of position......must be 'englufed' by main flame when it lits
Ceramic insulator....cracked & tracking to ground. New electrode
Dirty/corroded/loose ground ----electrode mounting screw provides ground...clean/tighten
High tension wire....corroded/dirty/loose ---clean up/tighten up
OR replace spark electrode with new