I created a fillable form using Adobe Livecycle, and save it with Adobe Reader extension.
I tested it in my computer, and Adobe Reader can open and fill the form without issues. Another colleague does not find any issues either. But when I send the form many people, already a few of them responded that they were not able to open the form in Adobe Reader. Please find the screenshot below.
It looks like they are trying to open it with something other than Reader. This message will show up if you try to open it on a smart device such as a smart phone or tablet. It will also happen if you try to open it in the browser. Ask the users to download the file than try to open it.