You kept your receipt? That'll show exactly when and where you bought it.
If you paid via Walmart Pay using the Walmart app, then you have a receipt tied to your Walmart account. Or you should have received a paper register receipt upon purchase. Or email, if you bought it online.
If you need service on a Roku TV, that would be either the store (Walmart) or the manufacturer (whatever the brand is). Roku does not build Roku TVs. Roku licenses their software to TV manufacturers for them to include it in the manufacturers' TVs. Roku TV Support page.
Roku Community Streaming Expert
Note: I am not a Roku employee.
If this post solves your problem please help others find this answer and click "Accept as Solution."