Best way to create a multi-table report

Hello everyone. Im still a newby at Power BI, so I am sorry if this is a silly question.

To sum up: I want to show a report with the ammount of times every type of transaction is done.

The problem is that I have the type of every transaction located in a different table. Example:

The report would use the count of Products.idproduct for each Transaction.Transaction type. Now, this would be easy if I could make a new table with only those registries, but unfortunatelly both Products and Transaction have a huge ammount of data and I want to avoid increasing the refresh/load times as much as I can.

As you can see in the (odd) example, Products and Transaction have one field in common, the barcode. Every Transaction.itemBarcode must exist in Product.productBarcode, maybe multiple times, but not every Product.productBarcode exists in Transaction.itemBarcode

I tried to make a relationship between those tables so I could use a DAX Function to retrieve data, but when I was about to create that new relationship this message appered:

I am not really sure about whats wrong there, but I can guess that it will increase the refresh/load time...

Right now I'm not sure about what should I do, so I wanted to ask you guys!

What would you do in my place? And, is there any way to do it efficiently?

Thanks a Bunch!