This is from the IRS directly :
Qualifying for an HSA
To be an eligible individual and qualify for an HSA, you must meet the following requirements.
You are covered under a high deductible health plan (HDHP), described later, on the first day of the month.
You have no other health coverage except what is permitted under Other health coverage , later.
You aren’t enrolled in Medicare.
You can’t be claimed as a dependent on someone else’s 2018 tax return.
https://www.irs.gov/publications/p969
