Your user needs to be merged with an LDAP-backed user, it says

[jidanni]

https://wiki.osgeo.org/ says

Your user needs to be merged with an LDAP-backed user.
If you do not yet have an LDAP user, you will need to create one before you can continue.

But when we click Next, it doesn't allow us to create one. I just asks us
LDAP-backed username and password.

[hexmode]

You cannot create an LDAP user with this process. You need to have one already. One can be created by following the instructions here: https://www.osgeo.org/community/getting-started-osgeo/osgeo_userid/

[jidanni]

OK, but that is never mentioned between
https://wiki.osgeo.org/w/index.php?title=Special:WikiUserMerge&returnto=Main_Page
and
https://wiki.osgeo.org/wiki/Special:WikiUserMerge/account .
Thus everybody will get stuck.

[jidanni]

Wait, https://www.osgeo.org/community/getting-started-osgeo/osgeo_userid/ says

Other Services
The following services have their own sign-on:

Wiki: The OSGeo wiki uses it’s own userid list for those wanting to edit, but it is easy to request and account for editing. Please make sure to add your email address to the appropriate field because it is used to verify you are a real person. Before that it is not possible to edit the Wiki.

So I'm guessing that's now wrong.