Romans 15:19-21 says:
{19} …from Jerusalem and all the way around to Illyricum I have fulfilled the ministry of the gospel of Christ; {20} and thus I make it my ambition to preach the gospel, not where Christ has already been named, lest I build on someone else’s foundation, {21} but as it is written, “Those who have never been told of him will see, and those who have never heard will understand.”
In this section of Romans 15, Paul is saying that he wants to preach the gospel message to people who haven’t heard it yet. The phrases ‘Those who have never been told’ and ‘those who have never heard’ refer to people who haven’t heard the gospel message; Paul wants to preach to these people so that they will ‘see’ and ‘understand’.