I have company health insurance, also an HSA. On Form 8889 it asks if I was covered by High Deductible plan in 2018 I was not, but it won't let me efile w/out checking

To contribute to an HSA plan, you must have been covered by a High Deductible plan -- that's why the program is seeing an error.

Please see TurboTaxDanielV's advice below.  He's correct -- you may actually be under an HDHP, otherwise you generally wouldn't have been allowed to contribute to the HSA.

https://ttlc.intuit.com/replies/6476383