Traditional IRA contribution

[ELM11]

I retired on January 15th. I made one contribution into my 401K before it was terminated and transferred to my Investment Adviser. At that point, I no longer was a participant in the 401K plan. Does that one contribution before termination disqualify me from contributing to a Traditional IRA and taking the tax deduction for doing so? 

[fanfare]

If the "retirement plan" box is checkmarked on your W-2, that's what counts.
BUT

if you are in the permitted AGI range,

you may still be able to deduct it.

Otherwise, it goes on Form 8606 or you reverse the contribution before Tax Day.

[MinhT1]

If you made even one contribution to your 401(k) plan in 2021, then you are considered as covered by a retirement plan at work in 2021. As you did not specify the year, I assume that it was 2021. But the same applies if it was 2022.

As your eligibility to contribute to a deductible IRA depends on your filing status and your Modified Adjusted Gross Income (MAGI) according to this IRS table.

[Opus 17]

Remember that you must also have compensation (income from working) to make any kind of contribution to an IRA.  You have some compensation up to your retirement date, and might have compensation from other jobs, but the compensation issue will also affect your ability to contribute to an IRA.