Can I correct a spelling mistake in a passenger's name after the ticket has already been issued?

Sometimes yes, but it depends on the airline rather than on your portal. Many airlines allow a small correction of a letter or two where it is clearly a typing error and the passenger is plainly the same person. Others treat any change to a name as a different passenger altogether, which means the ticket has to be cancelled and a new one issued. Your portal can send the request, but it cannot overrule the airline's own rule.

The single biggest factor is how quickly you spot it. Most airlines allow a ticket to be voided within a short window after issue, usually the same day, and if you are inside that window you can void and rebook with the correct name at no loss. Once that window closes, you are into a reissue, which can carry an airline fee and a fare difference if the original fare is no longer available. Low cost carriers work differently again and often treat a name change as a chargeable service rather than a correction.

How it works in practice also depends on the supplier connection behind your flight search. Some suppliers let a correction be requested straight from the booking record in your portal, while others need it raised with their support team because airlines do not all allow name changes to be made automatically. Either way you will see the booking and its status in your own system, so you are not chasing it blind.

The practical habit is to read the passenger names on the confirmation against the passport the moment the ticket is issued, and if anything is wrong, raise it the same day while the void option is still open.