What does this mean?
This is a check against the HMRC pattern for page title.
In the pattern there are 3 or 4 parts and this error is generally thown if you have omitted one or more.
<title>Page Heading - Section (optional) - Service Name - GOV.UK</title>
There is also a related issue around a missing error prefix.
Impact on users
The page title performs an important role in telling users where they are, especially if they have several tabs open at once.
For screen-reader users it is especially important to include all the information in the pattern as they use it to confirm they are on the correct page before starting to explore the page.
How to fix
Check your page title against the pattern.
If your page has an error state check that the "Error" prefix is displayed at the start of the title when the errors are shown on the page.
Note for internal services there is no need for the GOV.UK part of the title as these do not sit on that domain.