My code is not M code (power query language) it is a DAX formula to calculate a new column in your sales table.
1. Go to the "Data" tab (icon looks like a table in the menu in the left side) (Don't enter "Edit Queries")
2. Go to your table with your sales transactions
3. On the "Home" tab in the top menu select "New Column" under "Calculations" (just to the left of the Publish button)
4. Insert this code in the formula field "FirstDayOfWeek = DATE([Year];1;-2) - WEEKDAY(DATE([Year];1;3)) + [Week_number]*7" (and press enter)
You now have a new column in your sales table the should hold the first date of the week calculated from your year and week column and you should be able to make a relationship between this column and your date column in your calendar table.
I would create a column in your 'Matas_Weekly_Sales' table that is the actual date (use a date time) for the start, or end of each week period the transaction belongs to.
Then create a relationship between this new column and the [Date] column in your 'Date' table. Do not try to create a relationship to the [Week_number] column in your table. It won't work and you don't need to 🙂
In your table with sales data you could calculate the first date of the week with using the formula below (replace the column references "Year" and "Week" with the names of the columns in your table), and then connect this column to your calendar table.
Your date table containes unique Dates but each week has seven days. So each week number appears seven times a year multiplied by the number of years. So week number is no unique value to create a relationship with.
This is why @sdjensen suggested you to add a Date to your table so you can relate this to the date column of your date table which contains unique values.