Short answer
If a property is not registered, the usual registered title documents may not be available from a standard address search. The next step depends on the region and the information already known about the property.
Official source: https://www.gov.uk/registering-land-or-property-with-land-registry
Why a property may not appear
A property may be unregistered, filed under a different address, split across more than one title, or recorded in a way that does not match the address entered.
Official source: https://www.gov.uk/get-information-about-property-and-land/search-the-register
What unregistered means
For England and Wales, GOV.UK says a property might not be registered if it was owned before 1990 and has not been mortgaged since. Different rules and records apply in Scotland and Northern Ireland.
What to do next
Check the address carefully, try any known title number, and consider whether a different search route is needed. For England and Wales, GOV.UK notes that a search of the index map may be needed if a property does not appear.
Independent service note
LandRegistries.com can help with document ordering where records are available, but it cannot create registry documents for an unregistered property.