Inescapable error

Make sure the option to highlight the fields is enabled (via Edit - Preferences - Forms - Show border hover color for fields (yes, it's a very bad label). The required fields should then show up with a red border. If the field is hidden there's nothing you can do about it, though. I would then report this issue to the author of the form.

One thing you can look for, before contacting the form maker, is whether there is a check box (like "accept terms & conditions", .that sort of thing). The form designer could have made a required check box, which could get this message, but with no highlighting.

[Moderator moved from Using the Community (forums) to Acrobat Reader.]

Do you have the latest version of FREE Acrobat Reader?

https://acrobat.adobe.com/us/en/acrobat/pdf-reader.html

If yes, this suggests that the form itself was not programmed correctly. 

Contact whoever created the form and tell them that it's broken. You can't use it.