Cannot delete a line I've drawn

I was having the same problem and the suggestion about checking layers didn't work since I have no layers.

I suggest that you give up.  You won't be able to fix it in the current version of acrobat.  But, you might have an easier time if you go to view and select "disable new acrobat".  When I stumbled upon that, I was able to restore my previous version of acrobat.  Now I can make my edits as I need.  I feel like I've stopped wrestling with an 800 pound gorilla plus I can find all the tools I need.

• Open the Layers panel by going to "View" > "Show/Hide" > "Navigation Panes" > "Layers."
• Ensure that the layer containing the line is visible. If it's on a locked or hidden layer, you may not be able to select or edit it.

Please share a screenshot or a an example of your PDF file.

Was this PDF produced from a scan?

Are you using the "Edit PDF"  tool and you are not able to remove that image object?

What happens if you use the "Comment" tool instead of employing the Edit PDF or Prepare Form tools?

Sounds to me like that line could've been produced as an annotation (or Comment) on that PDF rather than an image object ( or a field object).

In which case, the Comment tool should facilitate the ability to remove such annotation.

Can you verify and confirm if this is the case?