linest function to develop a model for forcasting current month sales

I want to use the linest function to develop a model for forcasting current month sales. I've started using the attached workbook using prior month sales and prior year sales but cannot get the function to work.

It began by just giving me a numerical value and now I get a warning about a circular reference which I don't believe I have.

Could someone please get this to work and show me what I am doing wrong. I need to understand how to use the function as I will be adding more or different variables to it.

I would also like to be able to get the t values for each variable so that I can tell if they are useful or not.

Sorry just realized I forgot to attach example with function which isnt working

I'm still trying to get this to work without any luck. I've read and reread the help files and believe I am doing it correctly but so far no luck.

can anyone get it to work on their machine?