Lazaros Mitskopoulos

Hello, I am trying to apply for getting a letter of confirmation of residence legalised (Apostille) through gov.uk, but I have the following problem. A step of the process requires me to search through a list of the documents that can be legalised, and I can't find anything for the letter of confirmation of residence. And it doesn't allow me to proceed further with the application if I can't find the document and select it. Is there no longer a service for this document? Do I have to go through a third party? Looking forward to any piece of advice, Best wishes