ERROR: Missing Cost Code for Item '...' The Cost Code Assigned in LMN Has Not Been Matched to a QB Service Item

The estimate you are trying to export has a work area that been assigned a cost code, but the cost code has not been matched to any QB Service Item.

Run step #4 on the LMN QB Sync again and make sure all cost codes in LMN are linked to QB service items  (a yellow exclamation will appear beside any unlinked cost codes)