You need to add the PostgreSQL JDBC Driver in your project as mentioned in search.maven.org.
Gradle:
implementation 'org.postgresql:postgresql:42.2.10'
Maven:
<dependency>
<groupId>org.postgresql</groupId>
<artifactId>postgresql</artifactId>
<version>42.2.10</version>
</dependency>
You can also download the JAR and import to your project manually.
NOTE: Compile as used in this answer is deprecated. Replace with implementation as per 3.