I bought the same plan and had the same problem. All I could find was a DTV support number and none of the people I got pushed around to knew anything about the streaming Sunday Ticket. After 30 minutes, finally got the right people. Took a supervisor to finally tell me that they are having problems with the NFL+ website AND the Sunday Ticket website. Said they hoped to have it fixed by the preseason openers.
Even though I got the same story about not being able to cancel, I did get into my profile and was able to cancel the whole thing. The credit showed up on my card within 48 hours.
Dodged a $167 bullet.