[GNC] Assign as payment missing

[Marko Vukovic]

I am trying to declare a transaction as an invoice payment by following the instructions found here: https://www.gnucash.org/docs/v4/C/gnucash-help/busnss-ar-payment1.html
The problem is that I do not see "Assign as payment" option when I right click on the transaction or any of its splits. I see Sort By, Filter By, Rename Page, Duplicate Transaction, Delete Transaction, Remove Other Splits, Enter Transaction, Cancel Transaction, Open Linked Document (grayed out), Jump to Invoice (grayed out), Blank Transaction, Go to Date, Split Transaction (grayed out), Edit Exchange Rate, Schedule, Jump to the other account options in the pop-up menu.
The version is reported as 
Version: 4.6Build ID: 4.6+(2021-06-26)Finance::Quote: 1.49
I run GnuCash on Linux. What am I missing?
ThanksMarko