GOV UK One Login informs me:

Thanks for contacting the GOV.UK One Login Support Team.

To keep your information safe, we cannot give you any more details about why your identity could not be proven.

To discuss alternative ways of obtaining the information you require, please contact HMRC directly: https://www.gov.uk/contact-hmrc

Kind Regards,

GOV.UK One Login Support Team

Methinks that a programmer has employed a testicular elevation manoeuvre during the execution of their duties. (Keep my information safe, HAH!🤩). Regards, Toodles. 

Good luck!

I need details I do not possess and a whole long list of options for non UK applicants to use.

There's usually a group of people on the beach at Dover who know what details can be used to populate option-fields in UK Government forms.

Try downloading that from the iPhone App Store.

Does it have the same issues as that from HMRC?

As for @transparent's and @jamespa's points, I agree that this is very lazy programming. However, and assuming it is an official HMRC app, what is rather more worrying is not that there was a lazy programmer involved but that such a basic and glaring issue could have got through the subsequent testing before release to the general public. Software development is a process rather than just someone writing some code and hoping it'll work, so a failure of an HMRC coder to follow Home Office directions about how to implement a Home Office function and then the failure of an HMRC user acceptance testing process to pick up a bug that will affect a significant proportion of the population seems like a development process that's unfit for purpose at almost every stage.

My experience of many parts of www.gov.uk has been a surprisingly cohesive and robust IT offering overall considering the breadth of scope, so a developmental issue as fundamental as this has more far-reaching effects than might otherwise be immediately obvious.