Hi!
I found a solution.
You just need to copy the IC2.cfg from your Singleplayer modpack into your server config.
The config is in "C:\Users\[username]\AppData\Roaming\.technic\modpacks\tekkit-legends\config"
Go to your server folder and paste it into the config folder.
# Enable generation of copper in the world
B:enableWorldGenOreCopper=true
# Enable generation of tin in the world
B:enableWorldGenOreTin=true
They set the generation to false. So can also edit your server config and change it yourself to "true".
I searched 1:30 hours for a solution and I found nothing on Google except your post so I decided to share it with you!
... and sorry for my bad english... I'm from Germany!