SQL Add Parameter in Variable

Trying to add a parameter in my variable.  Having an error after the comma.  Says - "Incorrect syntax near Convert expected vairable".  Also receiving an error on VARCHAR, states expecting select.

DECLARE @msg VARCHAR(50) = 'The part number is not the correct length.  The length should be {0} ', CONVERT(VARCHAR, @PartLength);  
IF @Length <> @PartLength 
    THROW 50001, @msg, 1;

Select all Open in new window

How do I resolve this?