Why can't the tower defense elf dragon king be upgraded?

The reasons why the tower defense elf dragon king can't be upgraded are that it doesn't meet the upgrade conditions, insufficient resources and game bugs.

1, does not meet the upgrade conditions: the tower defense elf dragon king needs to kill a certain number of monsters or collect resources to upgrade, and cannot upgrade unless the corresponding conditions are met.

2. Insufficient resources: In tower defense game, upgrading needs to spend corresponding resources, such as gold coins or crystals. If the resources in hand are not enough to pay for the upgrade, then the upgrade cannot be carried out.

3. Bugs in the game: Bugs or program errors in tower defense game will also lead to tower defense upgrade failure. At this time, you can try to restart the game or contact the game customer service to solve the problem.