Instagram “error” even though username and password are correct

That is an error message provided by Instagram, not Apple. Some Meta users have found it necessary to delete Instagram and Facebook if you have it, and download them again. Otherwise Meta is your best source for problems with their apps. They don't have the greatest support options and do not have a Support Forum for their users to offer any help, but that is Meta's choice.

https://help.instagram.com/

carlsuness wrote:

https://discussions.apple.com/content/attachment/cf95b407-7fa9-49dc-a967-05e10373a8fc

[Edited by Moderator]

META support can help you out.