The way that I understand it, disobedience to Canon law is a mortal sin. Thus, failure to perform the Friday Penance can be equated to failing to attend Mass on Sunday or a Holy Day of Obligation.
Now let’s compare the two. If I forget to set the alarm clock and oversleep and miss Sunday Mass, then there is no sin because I did not do it intentionally. But if I then wake up and there is a later mass that I can go to but I don’t do so because I want to go to a fireworks display or go on a picnic or something like that, then I have commited a mortal sin by intentionally not attending mass.
Now, let’s compare that to my situation with the Friday Penance. I forgot about it. No sin. But then when I remembered, I should have made the time available to perform another penance in it’s place – skipped the fireworks display and spent the time in reading the bible or saying a rosary or left a family gathering early or arrive late or whatever.
Just as I am expected to take what ever reasonable means are available to me to attend Mass on Sunday and not pick a fireworks display over mass, I am also expected to make what ever reasonable means are available to me to perform the Friday Penance.
I did not give it the importance that was due and failed to perform the Penance – thus a mortal sin of omission which I attempted to confess.