The item ID for Wood wall 26 (inverted) is wood_wall_roof_upsidedown
This is the spawn command to give yourself Wood wall 26 (inverted) in Valheim normally without the level option.
Copy the command below and paste it into your Valheim game or server admin console to obtain.
This is the spawn command to give yourself Wood wall 26 (inverted) in Valheim with the level option if applicable. Below is just an example with the item chosen. Some items do not have leveling.
Copy the command below and paste it into your Valheim game or server admin console to obtain.