First off, great job – for someone relatively new to Power BI and data modeling, you put together a really nice star schema compliant data model that should be very well suited to your analysis. Also, thanks for providing all the relevant information and files – makes providing good support much easier.
OK, on to your problem. This is an absolutely textbook granularity mismatch problem, frequently seen in budget contexts comparing actual to budgeted amounts, and also in cases similar to yours comparing actuals to forecasts.
What you need to do is to allocate your weekly forecasts down to the daily level, such that your Dates table, your inventory and forecasts are all operating on a daily granularity. There are two different strategies you can use to do this – the first is to do it via DAX and then build the necessary relationship in your data model virtually using TREATAS. The second approach is to do the allocation in Power Query and then connect your tables using a physical relationship, the same way you have for all other relationships within your data model.
Please refer to the granularity section of the following post, where I provide links on exactly how to implement both methods. Both methods are entirely valid and the choice will be one largely of personal preference. Personally, I prefer the Power Query approach , which I think is simpler and more intuitive.
I hope this is helpful. If you have any problems in implementation, just give a shout.
– Brian