System.Net.Sockets are not supported

Today I wanted to build my game to iOS for Xcode, and got this error:

Error building Player: SystemException: System.Net.Sockets are supported only on Unity iOS Pro. Referenced from assembly ‘Assembly-CSharp’.

How do I remove this error so i can finally get a successful build?

Sockets are an iOS Pro feature only.

Check your scripts for sockets usage. In case it’s a third-party library you’re using, contact the package vendor.