Check if the subscription was successful
Okay, I know it sounds dumb because you got the activation code released only after a successful payment but there’s a probability. Use your phone or computer to go to www.espn.com, log in to the same ESPN account that you used for subscriptions and check if the ESPN+ (monthly) subscription is live or not. You should be able to pull off some clues from here.
Are you using the correct procedure?

It’s pretty important to make sure you are using the correct procedure for purchasing and linking your streaming device to the ESPN+ account. We at GetDroidTips have a guide on how you can activate ESPN on Roku, Amazon FireStick TV, Xfinity, Apple TV, Hulu, Samsung Smart TV, and LG Smart TV that you can use for subscription and linking the device.
Check if you have missed any instructions. Move to the next method if this doesn’t help you.
Remove channel/app
On any Smart TV or streaming device, you can use ESPN+ either as a channel or app, right? Since you are facing difficulty authenticating the activation code, why don’t you remove the channel/app and check if that makes any difference? Based on which streaming device you are using, the method will of course differ.
Once you have removed the app/channel, make sure to restart the system and then attempt reinstalling ESPN+. Check back if you can use the activation code provided or not.
Talk to the Service Provider
Assuming that there are many ways to get ESPN+ and one of them is via service providers, they should provide you with the correct activation code. If you purchase a Verizon plan with ESPN+ included but fail to authenticate ESPN Plus, it’s time to talk to Verizon customer care. Keep certain screenshots such as the error message you get after feeding the activation code handy.
If the problem is from the service provider’s side, they should be able to come up with a resolution.
Talk to ESPN Support
This is a no-brainer. If the problem is specific to ESPN Plus which is a third-party app on your streaming device or TV, you will have to contact ESPN Support for help. The folks will be able to update you on the situation and take appropriate actions to resolve the ESPN Plus activation code not working issue. Visit here for ESPN Support.
Note that you need to use the same login credentials and keep the subscription details handy in case the support team member wants to authenticate the purchase.
Why am I not able to watch ESPN ?
I keep getting prompted to input a code and it doesn’t take.