Entity Framework Core Visual Studio Code : Error when adding migrations

Error
Could not execute because the specified command or file was not found.
Possible reasons for this include:
  * You misspelled a built-in dotnet command.
  * You intended to execute a .NET Core program, but dotnet-ef does not exist.
  * You intended to run a global tool, but a dotnet-prefixed executable with this name could not be found on the PATH.

Solution:
Make sure .NET Core is installed and added to environment variable path.
https://docs.microsoft.com/en-us/dotnet/core/tools/troubleshoot-usage-issues