New-ish transmission going out.

So I bought a b series trans in 2021 aug. the AT stutters, and I only drive city sometimes the freeway but I took it to a trans mechanic and he said drive it on the freeway(haven’t got to do that) the code pops up around 50 mph. And I noticed today at the red/green light the tranny stuttered as I gradually step on the gas from 1st to 2nd . I bought the solenoid that goes right below the coolant pipes. Hopefully that would be case🤞 ha.