Invoice dropdown is disabled when adding new revenue

I created multiple invoices for one customer and i am trying to add a revenue for a specific invoice but when i add new revenue the invoice option is always disabled, how does this work?

Where are you trying to create the new category, while entering the Invoice or by using the Settings->Categories option?

Also, when you say "the invoice option is always disabled", where are you seeing this? When creating a new Category you have the following Type options: "Income, Expense, Item, and Other" so I am confused as to where you are having the problem.

I am also having the same issue as Usman. An example is here
https://postimg.cc/n9KdF5nm

The only way to create a revenue and assign it to the invoice is to go on to the invoice and click the "Add Payment" button.

Hi I have the same problem.
In the Revenue section (under "Sales" in the menu):
- Category: Sales (so income, that should be ok)
- Bank account, Customer and payment method are choosen.
- Sum is exactly the same as the invoice total.
- Still the invoice dropdown stays greyed out.
The same problem for the Bills sections: I can't link an outgoing payment to an existing Bill.