If you have no cards in SafeInCloud database for this website, then the extension will offer you to search for the card in the existing database or add a new entry.
More details on how to use can be found here: https://safeincloud.ladesk.com/340234-Register-new-account
If you already have a card for this website in the SafeInCloud database, make sure that this card has enabled the Use for Autofill checkbox. This card must also have non-empty Login and Password fields, and a Website field that matches the address in the browser. The exact match is not required. They only have to have the same domain, e.g. online.domain.com matches www.domain.com.
If the error still remains, press the Edit button at the bottom of the card. In the card editing window press the Autofill icon on the toolbar.
In the appeared Autofill dialog check that all the fields have correct autofill attributes (like on the screenshot below).