Hello Laura,
the collective search help that is behind this field is MPLA_ES (Structure EAMS_S_SP_MPLAN_ID -> Field EAMS_WARPL).
This error means that none of the elementary search helps ( MPLA_ES_SIMPLE_E nor MPLA_ES_ADVANCED_E) are working.
Both of this elementary search helps read the data from the enterprise search, and in order to make them work, the enterprise search connector for maintenance plan should be active in your system.
Depending on your release, this search connector will be MAINTENANCE_PLAN_H, if you are using HANA or BUS1020 if you are using TREX.
Creating the search connector will repair your problem.
Best regards,
Julián