As
@Seym notes, the instruction to list additional countries visited in the "Reason" box has been in the Online Presence Calculator instructions a long, long time.
BUT as mistakes and oversights go, this is NOT a biggie. If it is the only mistake you made, congratulations, you did a fantastic job. Really.
You could use the webform to submit a correction, but there is no rush . . . And, again, if this is the only mistake you made, even doing that does not seem necessary. Typically the worst case scenario for an omission like this is the applicant might be asked about it during questioning attendant the testing or interview.
In fact, where this tends to come up as a question is when there are passport stamps showing the trip to a country not listed in the applicant's travel history. Unless there are some other collateral circumstances suggesting reason to be suspicious about the applicant's credibility, it really is NOT a big deal, not at all. "
Oh, forgot to include that side trip." (After all, it does not change the calculation of days at all.)
There are many reasons underlying why IRCC asks for this or that information, so usually does not help much to speculate about this or that precise reason (there are typically multiple reasons for why IRCC wants this or that information). But it is probably safe to say that at least one of the reasons why IRCC wants the applicant to disclose additional countries visited during periods of time abroad is just so they can more easily compare and reconcile passport stamps with the information provided by the applicant.
So unless the applicant was actually trying to hide having gone to this or that country, it really is NOT anything to worry about.
Yeah, I use a lot of words to say "
No Need to Worry."