In previous versions (admittedly I don't when this stopped) there used to be a Credentials link in the left hand menu of Jenkins. The link is no longer there at either the root level or within folders.

Adding /credentials/ to the end of the URL works as a workaround but without knowing that then there doesn't appear to be a way to get to the credentials that are set at folder level.

Attached are two screenshots, one showing what I see now and one found from a Google search of what the menu item used to look like.

I've made an assumption about the components involved but they might need to be changed.