Hi, you have run across one of the "issues" in databases in Ancestry.com.
This is that there is very inconsistent encoding from one database to the next, and if you haven't used a particular database before you won't know how the code script will behave.
There are quite a few databases that are not encoded to automatically create a spouse, enter the date, and read correctly in the "event": date, place, and spouse in this case.
You can't edit them to add the name of the spouse.
For some of these, if you create the Marriage Event via "add event," and ~then~ add the database as a "source," it will automatically attach as a source for the Marriage you created. But my experience is that most don't.
All is not lost, however.
Go ahead and create the marriage event with date, person, place.
You will still only have the Marriage Event Citation attached to one party (the database was not encoded to create the event for the other party to the marriage, right?)
For the person the Citation attached to, click on "edit person" link in the large box. Click on "Relationship Events" tab. At the far right across from "Marriage to Unknown" click on the tiny icon next to the gray "more options" button.
This will show you a drop-down below the listing for the marrige; click on the underlined link. This takes you to a barely useful Source Citation page -- at right is the option to edit or delete. Click to "edit". On the edit page you can fill in the empty boxes if you wish, but scroll all the way down to the listing of what the citation is attached to. If you click the tiny arrow you get a drop-down list of all Timeline Events for the person -- here you can switch the source-citation from Marriage To Unknown to the "Marriage To XY" event that you created without having to manually enter the source citation.
Click to save, and you can return to the person's Overview page. You can now delete the "Marriage To Unknown" event without disturbing the source-citation. You do this by clicking on the title of this event, then scrolling down to where the Delete link is next to a trashcan.
Now go to the overview page for the spouse that you manually created the marriage for, and look to see if the same source-citation appears for the spouse's marriage to the first person. With luck, it will -- you can tell for sure by clicking on the title of the Event, then scrolling down for a look at whether the source you wanted to add is listed at left below data on the marriage.
If the source-citation was not added on the spouse's Marriage Event (which really should be a duplicate of the one you just fixed), (grrrrr) you will want to search for the marriage source-item ~from~ this person, add it (there will be an added event, Marriage to Unknown). Then switch what Marriage Event the source-citation is linked to as you did above, and delete the Marriage to Unknown.
It's pretty clicky and annoying, but goes pretty fast after you've been through it once.
It is unfortunate that we cannot edit or otherwise change the titles of Events. The software has limitations -- as do the databases.
Be aware that anything from the Millenium File or entitled "US and International . . . ." is from family group sheets and mostly-junk genealogies and should be considered to be trash unless you can verify that an actual record exists. If you find an actual record, delete the trash-source and manually enter a citation to the real record.
If you'd rather not add trash to your tree (which could include adding a wrong spouse in this case), you can write a note about the purported marriage in the "comments" section, including your own thoughts about why the reference may or may not refer to an actual marriage record "as yet not found."