Got this error when trying to place linked belts by dragging them, as I couldn't figure out how to get them to link. Could you put that info in the tooltips, as well as adding that hotkey/GUI/hotbar option that other guy suggested? Btw you didn't add proper names to the fast and express linked belts, it just has "unknown-key: 'entity-name.fast-linked-belt'"

https://i.imgur.com/WMS7qyy.png

As an FYI, the error happened before I updated to version 0.0.3

Update: turns out it happens in 0.0.3 as well, I had a pair of express belts down and linked properly, and when I placed the third one down, the game crashed.

https://i.imgur.com/keJeV0i.png

It seems to be attempting to link the third linked belt to the previous pair, it happens no matter what tier of linked belt is already down or which tier I'm placing.

Another update: I'm the host(we're playing Space Exploration + Krastorio 2 and a bunch of other mods), and one of my players seems to be able to use them just fine. Whenever I place down a third linked belt, no matter the tier, the game crashes. My current pair of linked belts is going from one surface to another, if it helps.

Yet another update: Confirmed, my players can all place linked belts, but I can't, not even when I switch to single player.

After backing up my save and removing the belts I'd placed, it's now operating normally. I can provide the save file with the weirdness if needed.

Please try new version.

how u link them

With the control.lua via API:
https://lua-api.factorio.com/latest/LuaEntity.html#LuaEntity.connect_linked_belts
https://lua-api.factorio.com/latest/LuaEntity.html#LuaEntity.disconnect_linked_belts
https://lua-api.factorio.com/latest/LuaEntity.html#LuaEntity.linked_belt_type
https://lua-api.factorio.com/latest/LuaEntity.html#LuaEntity.linked_belt_neighbour