I’ve seen several of these so it may be helpful to make a post on it.
Some people who have not purchased Warcraft 3: Reforged are finding that with the update, they can’t get into the game at all, not even to play Classic mode. In order to play Classic without buying Reforged, one has to own the original Warcraft 3.
Meaning, you must prove to Battlenet that you indeed do own the original WC3. And that is done by linking your Reign of Chaos* game key to your Battlenet account.
To do that, go to your Account management and enter the key into the “Redeem a Code” field. After the process is complete, confirm that the key is listed under your “Games & Subscriptions”>“Classic Games”:
*Note: Linking only a RoC key will grant both Classic RoC and Classic TFT in the Reforged client. However, linking only a TFT key will do nothing. In fact, TFT keys are currently no longer needed for anything.
If for some reason the website won’t accept your RoC game key, contact Customer Support directly via a support ticket. I recommend either the Live Chat or Callback if they are open.