I don't think you can do co-op on two different platforms. You see, each platform gets updates at a different rate, so playing on different devices would throw the whole system out of whack.
You could, in fact, do co-op on two different platforms, but this is
only works if the players is on mobile (Android/iOS) and PC (Windows/Linux/Mac). Platform difference is probably one of the bottlenecks that could cause any problems, so it's advised to play on the same platform for multiplayer.
I WANT TO PLAY CO OP ON PC AND MOBILE AND THE SERVER IS ON PC BUT NOTHING SHOW
...CAN EVERYONE HELP ME ?
You can play co-op on PC and Mobile through the same WiFi, but multiplayer mode is disabled by default on Mobile so you must activate it first. You can read about how to activate it here:
https://stardewvalley.net/mobile-nultiplayer-guide. But if you're not on the same WiFi (a.k.a. online multiplayer through the Internet), you could try MeshNet feature from NordVPN to establish virtual network that behaves like local network. You can read more here:
https://meshnet.nordvpn.com/how-to/gaming/stardew-valley.
Hope this helps, feel free to ask anything.