Type 'System.Text.StringBuilder' is missing from assembly

[J0ska]

I have tried solution from this post with no success

Legacy-To-Wondows: 'System.Net.Mail.MailMessage' is missing from assembly 'System.Net.Mail

Getting the error post Legacy to Windows migration, how can we fix this? Validation Error BC31424: Type ‘System.Net.Mail.MailMessage’ in assembly ‘3bcd00f4-a916-40f9-b663-68496de3fbfd, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null’ has been forwarded to assembly ‘System.Net.Mail, Version=6.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51’. Either a reference to ‘System.Net.Mail, Version=6.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51’ is missing from your project or the…