I'll add a few cursory sources I found that claim to confirm this

these commands do not work on servers. only single player. even if you are an admin of said server.

As other people have said, devcommands do not currently work on dedicated servers.

Depending on what you are trying to achieve, you can download the world files from your server ( make sure you stop the service first) to your PC's valheim world folder, and start up the same world in non-server mode and do whatever you wanted to do from the console ( spawn in mobs, build in creative mode etc...) and then exit that game, and transfer the world files back to your dedicated server, and re-start the service on the server.

Quite a faff, but depends what you need to do an why, as to whether it is worth if for you.

I have seen a Dev comment, that they were not expecting dedicated servers to be so popular, so they haven't implemented features like devcommands. That may change however given how popular hosted servers have proven to be...(and obviously no timescales for when they may implement those features)

- Reddit thread

Cannot Be Used on Dedicated Servers.
Cheat console commands cannot be used on a dedicated server. Even when playing in single-player, if you join a dedicated server using the Join IP option, you won't be able to use the cheat console commands.

- game8.co article[game8.co]

Once in the game, to activate cheats, type devcommands and press enter (previously, the command was 'imacheater' but that has changed). Then you'll be able to use the following cheats in singleplayer mode. They won't work on a multiplayer server.

- PC Gamer article

Maybe this changed recently and I missed it?