As Bill suggested debugging is what every developer must learn in order to know what is happening inside when the programme does not behave as expected.
Please let me give you some more suggestions.
The first problem is
Console.Write("Enter degrees Fahrenheit to convert to Celsius OR
\nenter degrees Celsius to convert to Fahrenheit.");
will sabotage a successful compilation.
You should replace it by
Console.Write("Enter degrees Fahrenheit to convert to Celsius OR\nenter degrees Celsius to convert to Fahrenheit.");
The next problem is
double tempscale = Convert.ToDouble(Console.ReadLine());
You are expecting the user to enter either 'C' or 'F'.
No matter what the user will enter you convert it to double which is not a suitable data type for characters.
You should replace it by
var tempscale = Console.ReadLine();
The lines to follow then make no sense.
Hint:
You can start with this for conversion from Celsius to Fahrenheit
if (tempscale == "C")
{
double degreeFahrenheit = 9 / 5 * (currentdeg + 32);
Console.Write(currentdeg + "degrees Celsius is " + degreeFahrenheit + " degrees Fahrenheit");
Then you must do something similar for the opposite direction.
BTW: Your formula for conversion from Fahrenheit to Celsius is wrong!
Moreover you must take care of error handling to catch invalid inputs e.g. the user entering other letters than 'C' or 'F' in the first Console.Readline.
What will happen if the user enters any other than valid numbers in the second Console.ReadLine?