EntityFramework is not part of the .NET framework. So a simple explanation is that the machine this fails on doesn't have it installed. You are supposed to deploy it yourself. When you use the Nuget package then you'll have a copy of the DLL in your bin\Release directory. Don't forget to include it with your shipping binaries.
Next failure mode is that you did deploy it but the working directory of the program is not where you hope it is. Provide the full path name of the assembly to avoid this. You can use, say, Assembly.GetEntryAssembly().Location to find the path of the EXE.