Why can't I file with an HSA and not high deductible plan?

Only individuals who are covered by a high deductible plan may contribute to an HSA.  That's just the rule. 

From IRS Publication 969

Qualifying for an HSA Contribution

To be an eligible individual and qualify for an HSA contribution, 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 2021 tax return.