Comments not working on new EE3 install

I have tried everything to get comments to work to no avail. I have reduced the problem to the simplest form possible:

1. I am logged in as the site admin.
2. I have comments enabled for the particular channel that the entry and the comment belong to.
3. For test purposes I have hard-coded the channel name and entry ID number.
4. The channel, and the channel entry both exist.

But I still get this message when I attempt to post a comment: “You are not allowed to post comments.” I’m the freaking site admin! If I can’t post comments who can?? My comment form code looks like this:

{exp:comment:form channel="wall_posts" entry_id="78" dynamic="off" }
    
    <textarea name="comment" cols="70" rows="10">{comment}</textarea><br >
    <input type="submit" name="submit" value="Submit" />

{/exp:comment:form}

Can’t get much simpler than that, right? But it I still get the same message. I’ve tried everything else I can think of. Please help!