To upgrade your wall using elixir instead of gold, 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. 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.
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.