No Relations found between two tables

You should combine the data from both of these tables into a single data table either before load (via a query) or during load (via Power Query - Get Data).  You should then have a  single data table containing date, desc, sales amount.

Then create and load a calendar table (sounds like you may have one of these already).  it needs to have the same granularity as the dates in your data table.  eg if your data table dates are "day level", then your calendar table must be the same.  Read my blog post about calendar tables here  http://exceleratorbi.com.au/power-pivot-calendar-tables/ 

Assuming you have a day calendar table, you can use inbuilt time intelligence to solve the problem.

Total Sales = sum(data[Sales Amount])

Total Sales prior year = calculate([total sales],dateadd(calendar[date],-1,Year))