I am a Tennis Channel/Tennis Channel Plus subscriber but recently have been unable to access matches.

Tennis Channel Plus is not part of DirecTV Stream service. It is a separate subscription and you can only get the regular Tennis channel programing with a DirecTV Stream subscription. 
You may need to sign onto the Plus programing app with the credentials from the service you originally purchased Tennis Channel Plus from, Roku, Fire Stick, apple TV, etc. 

There is 2 sign in's to the tennis channel.  one for the standard tennis channel, which requires provider validation, and for tennis channel plus, which requires a subscription and a separate login.

I have a similar issue where I get the code on the screen to sign in with the provider, go to HTTP://tennischannel.com/activate  type in code, select DirecTv (I am DirecTv stream customer) log in with my DirecTV/att credentials, and the app on my Ruko pops up that customer is not authorized. I have the same issue with an Apple TV as well. 

I can sign into Tennis Channel plus without issue. 

@rwfowler I think I found the issue... DirecTv Stream is not supporting the watching of the tennis channel via the tennis channel app. 

https://www.directv.com/support/stream/article/KM1222524/

The Tennis channel is not listed... So to watch the tennis channel you need to use the DirecTV app. This would explain why we get the "not authorized" pop-up message.

Thank you lu do and ttuir97 for your assistance. Much appreciated!!

DirecTV Stream does support "The Tennis Channel". You just have to watch what content you select from the app. Selecting a "Plus" content will give you an error. 

@bcbsncjlj  I think you are missing the point.  trying to authenticate the "provider login" does not work on the Tennis Channel App.  I think it's a tennis channel issue, not a DirecTv Stream issue.  

This is the screen if you want to watch Tennis Channel via the Web:  I can authenticate no issues and watch the Tennis channel. 

Load app. Select watch "The Tennis Channel". Ask to select provider. Select DirecTV Stream. Login. Go back to app and select watch "The Tennis Channel". Done! Can't watch a PLUS content. Provide screen below.

Just installed on 2 Fire Sticks and 1 Fire TV stick. No issues authenticating. Sorry but I am traveling and did not bring a Roku stick. Roku is a pain authenticating WiFi on the road especially in a hotel.

@bcbsncjlj  I am an (Edited per community guidelines). if you click on search providers and type DirecTV, DirecTV Stream shows up. Selected that and provided my credentials and it worked...

What was confusing me was the DirecTV vs DirecTV Stream. I swear I have authenticated on other apps using DirecTV and it has worked...  Also what is a bit strange is that on the tennis channel's watch on the web, the first one is DirecTV Stream.. odd they would not align... 

Anyway, I am all good now. Thanks for the help!  @rwfowler I hope this solves your issue as well. 

All that matters is it works for you now. Enjoy and have a good day.

Thanks, bcbsncili!

(Edited per community guidelines) Fowler

rwfowler1752@gmail.com