How to fix error "Cannot load PowerShell snap-in Microsoft.PowerShell.Diagnostics because of the following error: Could not load file or assembly"

I'm trying to execute a Powershell script from an asp.net webpage and I keep getting this error when it tries to execute the script. I have tried 2 methods of executing the script and both give me the same error. The full error is:

An unhandled exception occurred while processing the request PSSnapInException: Cannot load PowerShell snap-in Microsoft.PowerShell.Diagnostics because of the following error: Could not load file or assembly 'C:\source\repos\WebApp\WebApp\Microsoft.PowerShell.Commands'. The system cannot find the file specified.

PowerShell powershell = PowerShell.Create();
            using (Runspace runspace = RunspaceFactory.CreateRunspace())
            {
                runspace.Open();
                powershell.Runspace = runspace;
                System.IO.StreamReader sr = new System.IO.StreamReader("C:\\Desktop\\test.ps1");
                powershell.AddScript(sr.ReadToEnd());
                var results = powershell.Invoke();
                if (powershell.Streams.Error.Count > 0)
                {
                    // error records were written to the error stream.
                    // do something with the items found.
                }
            }

    using (PowerShell PowerShellInstance = PowerShell.Create())
                {   
  PowerShellInstance.AddScript("C:\\Users\\RSpotton\\Desktop\\test.ps1");
                    PowerShellInstance.Invoke();
                }