Hey Ra33y,
First of all, welcome to callout pack development! I wish you all the best of luck and it's wonderful to see new people trying to make callouts.
That aside, let's look at your error. I'm presuming you tried following a tutorial and misread one of the steps. When creating your Project within Visual Studio, you'll want to look for Class Library (.NET Framework). What you did is choose Class Library which is not the same as it is not built on the .NET Framework.
This means you will need recreate your project, making sure to choose the correct option!
I hope this helps. Please let me know if you need any further guidance! ✨