The online message explaining that non UK residents are unable to submit their Self Assessment tax returns online is correct. As the message goes on to say, you can either,
(a) print off the relevant Self Assessment tax return pages and post them to HMRC
(b) you can use one of the HMRC-approved commercial software packages to submit one or more elements of your tax return online.
Self Assessment commercial software suppliers
Tax return for Self Assessment
Self Assessment: UK property (SA105)