Help me Lua

Error Tys to Tupac for fixing.

[ERROR] gamemodes/underdone/entities/entities/underdone_billboard/shared.lua:17: attempt to call method 'OpenPage' (a nil value)
  1. unknown - gamemodes/underdone/entities/entities/underdone_billboard/shared.lua:17
   2. Spawn - [C]:-1
    3. v - gamemodes/underdone/gamemode/core/sharedfiles/sh_propspawner.lua:50
     4. unknown - lua/includes/modules/hook.lua:84

you are calling ENT:Initialize() in a shared state, meaning it’s being called on the server and the client.
ENT:OpenPage() is a client-side function, so the server tries to run it, and it errors because it doesn’t exist.

Check if the current state is client before running ENT:OpenPage()

if CLIENT then