Recently I started exploring Power Apps in more depth. Its a nice tool but not without some issues. My scenario was to create a power app based on a SharePoint list which uses a custom content type. All the fields in the content type were optional. But still when I tried to save the form, it gave classic “An entry is required or has invalid value” exception.
Generally this error means that a required field is missing a value but I have no such fields in my content type. Also, strangely, I was able to save an item using the default new list item form.
Update (July,2018): I tested this again and now even the default list form throws error.
All the fields in the power app has “Required” property set to “false”. All the fields in the custom content type were “Optional” and I had only one content type present on the list.
After wasting almost 2 hours on this, I realized that power app, unlike default new list item form, does not take content type “Required/Optional” attribute into account. It gives preference to column settings. So in my case the erroneous field was “Title” column. It was optional in content type but not in column settings.
As soon as I set this to “No” in the column settings, my power app worked perfectly. You might have some other column like this, so please check.
P.S. I encountered the same for Power Apps list forms.
Hope this helps!