NCIS is definitely back on for Season 18, along with NCIS: Los Angeles and NCIS: New Orleans, although no premiere dates have been shored up yet — in part due to lingering complications from COVID-19.
Plans for shooting new episodes are still up in the air (although there are hints that California will start allowing productions to resume in July), which means that for the time being, stars of CBS' procedural have to find ways to keep themselves busy (and safe) until they get the call it's time to start working again.