Walls must be at level 8 or higher to be upgraded using Elixir. Additionally, if you'd like to use Elixir to upgrade a whole row of walls, they too must all be Level 6 or up. If there are any walls in the row that are at level 5 or lower, only gold can be used to complete the action.

If you're trying to upgrade a row of wall tiles and have no upgrade button at all, it means you don't have the storage capacity (nor the resources) to complete the upgrade, or the required Town Hall level. You first need to boost your resource storage and Town Hall levels, and collect enough gold or elixir to complete the upgrade.

Let me know if you have any other questions about how to enforce your walls, or anything else related to Clash. I'll be happy to help.

Clash on!