Why i cant play Adventure mode in Diablo III ,
I already bought the Diablo III Reaper of Soul package, but the game is still locked, why is this, please help me
Can you create a crusader hero?
If not, your RoS is not activated.
Have you redeem the authentication key (game key) yet, if you bought a physical copy.
If you bought a digital copy by credit card recently, it may take a few days for payment to be settled.
If you have questions, you should enter a ticket to contact Account/Payment support. Bug Report forum isn’t the correct channel for such issue.
I’ve played on all 3 Regions, so I can’t test this now. But, if I remember correctly, even in Seasons, you must have a least 1 Character that’s completed the game in Campaign Mode before Adventure Mode is unlocked in non-Season or Season.
When Seasons and Adventure Mode were first introduced, most of us had already completed the game… many times. Typically, Adventure Mode is automatically unlocked in Seasons. If it is not, you may need to play your Seasonal Character through the Campaign once. You can do this on Normal Difficulty to make it easier and quicker. Then, you can switch to Adventure Mode and complete your Season Journey.
Forgive me if I’ve remembered any of the above incorrectly.
If that’s right, it’d probably be quicker for the OP to make a non-seasonal hero and ask someone in General Chat in-game to take them to the Malthaeal quest and kill it with them to unlock access to Adventure Mode. Certainly a lot faster than completing the whole Campaign, even at Normal difficulty, on their seasonal.
I was going to mention that. But, they only have 2 low-Level Seasonal Characters. I wasn’t sure if they had Friends yet who could help them with a Malthael kill. I didn’t think about just asking in General Chat. 
And they don’t need to create a non-Seasonal Character. They can do it with their main Seasonal one. 
Remember that the hero taking them to the Malthaeal kill quest has to have completed the Campaign. Almost no seasonal heroes meet this criteria, but most people will have a non-seasonal hero that’s done so. That’s why I suggested a non-seasonal hero.
Anyway, this is getting off-topic, as this isn’t a bug.