The error message is produced by an outdated version of CheckStyle.

According to the CheckStyle ticket Support for Java 15 Strings you need at least CheckStyle version 8.36 if you want to use text blocks / multiline strings together with CheckStyle checks.

The error message (org.apache.maven.plugins:maven-checkstyle-plugin:2.17:check) indicates that your project uses the Maven-CheckStyle-Plugin version 2.17 which according to Maven Checkstyle Plugin Releases History by default uses CheckStyle version 6.11.2

You can try to configure your project to use the Maven CheckStyle Plugin 2.17 together with the CheckStyle version 8.36 using the following configuration (source: Upgrading Checkstyle at Runtime):

<project>
  ...
  <build>
    <pluginManagement>
      <plugins>
        <plugin>
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-checkstyle-plugin</artifactId>
          <version>2.17</version>
          <dependencies>
            <dependency>
              <groupId>com.puppycrawl.tools</groupId>
              <artifactId>checkstyle</artifactId>
              <version>8.36</version>
            </dependency>
          </dependencies>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
  ...
</project>

It's explained in the JEP. Emphasis mine

java.base exemptions. Since this feature uses a core library feature (java.lang.invoke) to implement a core language feature (String concatenation), we have to exempt the java.base module from using the indified String concat. Otherwise a circularity happens when the java.lang.invoke.* machinery requires String concat to work, which in turn requires the java.lang.invoke.* machinery. This exemption may limit the performance improvements observed from this feature, since many java.base classes will not be able to use it. We consider this an acceptable downside, which should be covered by the VM's optimizing compilers.