I need to send a invouce for only 50% of the total value. Do I have to create 2 invoices or I can add a note to the client to only pay 50% of the total of the invoice?

It works better to use two invoices

though you can use the note idea, many customers have issues when making a partial payment

You can also use progress invoicing with works like this. STep 1: Create an estimate Step 2: Convert to invoice (enable 50%) due Step 3: When it's time to invoice the remainder open up the estimate and convert to a 2nd invoice with the remaining balance 50%. This feature of progress invoicing has to be enabled in the account and settings (Gear icon) and is currently available with QBO Plus and Advanced subscriptions. Another option is the customer can just pay online with entering a custom amount in the pay amount field. It will be a live invoices online with QB Payments so the next time to go to pay online the balance will be updated using the same invoice link.

QuickBooks makes it so hard for us to understand and use this service. They are the worst, they seem not to know how to make this system easy. 

They are so hard that they even have to teach you and yet you still have to come to pages like this or look on google for answers. And then, out of nowhere they change everything, the layout, the buttons, everything without a notice. I wish someone will tell me a better system that way I can stop paying these guys for this cr@p. 

I believe thousands of people have this same problem. We have jobs that required some sort of down payment and this guys don't even put something to help us with it. Is it so hard to add something that says like 50% due on (certain date) and then remaining balance due on (a different date)

Please guys get your things together I am certain that you've lost thousands of clients because your system simply sucks.