I made a deck of 21 cards to study in my free time. I want to isolate the number of cards to one so I can study a particular card in one study session. I have Anki Version 2.15 installed on Ubuntu 20.04.
It’s not completely clear to me what you mean by “isolate the number of cards to one”. Besides, 2.15 is starting to become seriously outdated. You didn’t get an update until now because most package managers (including apt, the one used by Ubuntu so far) have failed packaging Anki since 2.1.16, so they were stuck to that version. I would recommend updating it, either by directly downloading Anki from apps.ankiweb.net or by installing a package manager that keeps up the pace. See a post I did about that.
I didn’t put enough detail. I want to isolate my deck from 20 cards to one so I can memorize a particular scripture. I reinstalled Ubuntu after getting locked up from changing my password and download my app from the app store. I have to use snap to upgrade.