With the most recent update to Clash of Clans, you are now able to upgrade your wall using elixir instead of gold — but, the wall tile needs to be at least level 6.
“In other words, the first upgrade you can buy with elixir is from level 6 to 7. If you’re upgrading a row, every single tile has to be at least a level 6,” Supercell says. “If there’s even one in there that’s level 5 or lower, the game will only let you use gold. Bring the lower-level tiles up to level 6 first, or remove them from your row, to level up the entire row with elixir.
Supercell explains, “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.”
In addition to adding the ability to upgrade walls with elixir, the update also added a ton of other new features including the new Lava Hound, level 7 Archers and Barbarians, Archer Tower level 13, and a bunch of other things you can read about here.