Problem with understanding Type mismatch error?

[u3rick]

Hi all

I am counting the days until Tsjallie returns from his vacation (He helps me on this project). In the mean time I am wondering if anyone can help me in my understanding of the Type mismatch Error? I understand that I need to match txt and numbers. I kinda understand that the data needs to come from like tables. I am wondering why I get the error on a seemingly simple sub as below;

Screen Shot 2016-03-21 at 11.34.42 AM.png


Screen Shot 2016-03-21 at 11.35.19 AM.png

Maybe someone could suggest some reading that I could do that might clear it up in my head?

I will attach the file if you want to take a look?