I found these pages on the Walmart site:
Walmart – Apple Services Offer
Walmart – Apple Services Terms & Conditions
Generally, if Apple was the one offering a free trial, it would only be open to people who had never had a free trial or subscription before. In this case, Walmart says that "Apple is not a sponsor of this promotion" (second link).
You say that you "don't have an active subscription at the moment", which leads me to think that you might have had a previous one.
From what I'm seeing n these pages, it sounds like only some returning subscribers may be eligible for the offer. It is not clear what the criteria for eligibility is for returning subscribers. A note on the first page says that you should contact Apple Support if you are unsure whether you are eligible for the free trial.