I just discovered and installed GNUstep SimpleAgenda on Debian Stretch.

Alt+n adds an appointment, Alt+t adds a task.

Both bring up their respective dialogues, but the OK button is always greyed out. I do fill in every field to make sure except the "Store" drop-down list, which won't open and I don't know what it does. The OK button is always greyed out, so I can't add tasks or appointments.

Anybody here familiar with this software can shed some light on the issue? I don't believe there is any manual. I couldn't find it.

(emphasis added)
I’m guessing it needs a calendar to manage...that is, a calendar on which to put the appointment or task. Guessing because the quote above is all the “documentation” I could find, unless the installation includes a man page.
I found that quote at https://packages.debian.org/stretch/agenda.app
So, my guess is that the store ddlb is to choose the “local or remote calendar” being managed...

EDIT: I found this
“SimpleAgenda requires the latest GNUstep stable release. The last version of libical that works with it is 0.48”
So mayhaps you also need to install GNUstep libcal...or use Thunderbird or Evolution, in which case you probably wouldn’t need simpleagenda at all.

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Silly me. I found the solution.

Click the WindowMaker/GNUstep style menu: Info -> Preferences -> Calendar Factory

Click the drop-down that probably reads "iCalendar" and select "local file."

Type the full path to the local file in the box right under it and click "Create."

Now it works. Thank you, scasey. Thanks to your guidance, I had a better idea of what to look for.

Solved.

I shouldn't have celebrated so fast.

It works until the application is closed. Then it forgets everything when it's launched again.

Upon inspection, it's clear that the local file was NOT created.

It gets worse. I can make it work again if I create the local file again... with a different name. If I try to use the same name, the "Create" button gets greyed out, which acknowledges that the file has been created before.

Except it hasn't. It's definitely not there.

I can repeat it many times and the greying out not only remembers what the last local file names was. It can remember all previously used names, and won't let me use any of them again.

So the only way to use this application is by running Preferences every damn time and "creating" a new local file, each time with a completely new name. Which will never materialize in the target directory.

Even then it will not work, of course, since the application forgets everythinng between exiting and launching again.

This program is broken and is unusable. Such a shame. I like its simplicity and old style looks.

Seriously...Thunderbird or Evolution.
The Thunderbird calendar will do everything you want locally...no need to link to an online calendar as I was talking about in my earlier post. No requirement to set up email, too, afaik.

I know nothing about evolution, but it’ll likely do the same.

Or, if you want to keep on with GNUstep, install libcal. I suspect (read: still guessing) there the “local file” needs to be some kind of calendar (ical?) file...but I’ll bet Thunderbird is in your repositories...