German pensioner in UK. No NINO. Cant do SA

I live in the UK. I am receiving a German pension and a small work pension from a German company. I have never worked in the UK so therefore, I do not have a NINO. I have no intention of working either . I have no assets or other financial gains in the UK . Because of the lack of a NINO, I cannot submit a SA online as a NINO must be supplied so I am unable to proceed. The German Tax Office in Neu-Brandenburg insists that I provide annually proof of no earnings in the UK in the form of a SA. I went through the online procedure of signing up for SA with No NINO . I printed out the form (SA1) and sent it to HMRC by post. No reply Online I registered for an UTR and was issued one. I also sent HMRC the Non EU/EEA certificate that the German Tax Office sent me that has to be stamped by HMRC and returned to me. No reply. I also inquired again by letter after I never got a response of the whereabouts of this form. No reply. So far, no response from HMRC apart from today, I receive a Penalty notice of £100 for failing to submit a SA. !! I am trying to be honest and transparent to both HMRC and the German Tax Office but am being penalised. Does anyone have knowledge in how I should proceed? Thanks

Hi,
We will need to access your record to review this for you.
You can contact HMRC by phone or webchat at Self Assessment: general enquiries
Thank you.