What the message says. You have requested an encrypted connection(*), but the SQL Server instance does not a certificate that you trust.
The best solution is make sure that SQL Server uses a proper certificate for the encryption, and that the client has this certificate (or a related root certificate) installed in it trusted-certificate store.
You can also set the option Trust Server Certificate=yes
in the connceton string. However, this makes you susceptible to man-in-the-middle attack. That is, someone pretending to be your SQL Server instance sits between you and SQL Server and corrupts or eacesrops on the traffic.
(*) In all modern APIs from Microsoft, this is the default setting since a few years back.