maven-compiler-plugin not found

Question

I am learning Selenium and I would like to try add the maven-compiler-plugin to pom.xml and reimport maven settings. So I found this example to do it http://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-source-and-target.html and tried to add the code to the pom.xml. But the vrsion from the example 3.8.1 is red like on the screenshot. What it means? It is a copy from example.

Here is the whole pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>camaj.vladimir</groupId>
    <artifactId>selenium-maven</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.seleniumhq.selenium</groupId>
            <artifactId>selenium-java</artifactId>
            <version>3.141.59</version>
        </dependency>
        <dependency>
            <groupId>org.seleniumhq.selenium</groupId>
            <artifactId>selenium-server</artifactId>
            <version>3.141.59</version>
        </dependency>
        <dependency>
            <groupId>org.seleniumhq.selenium</groupId>
            <artifactId>selenium-chrome-driver</artifactId>
            <version>3.141.59</version>
        </dependency>
        <dependency>
            <groupId>org.apache.poi</groupId>
            <artifactId>poi-ooxml</artifactId>
            <version>4.1.1</version>
        </dependency>
        <dependency>
            <groupId>com.google.code.tempus-fugit</groupId>
            <artifactId>tempus-fugit</artifactId>
            <version>1.1</version>
        </dependency>
        <dependency>
            <groupId>com.codeborne</groupId>
            <artifactId>phantomjsdriver</artifactId>
            <version>1.4.4</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Answer 1

In my case, I have changed the tags from plugin to dependency like it is in the Maven Repository.

<dependency>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.8.1</version>
</dependency>

Answer 2

In my case the groupId for the plugin was missing:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.8.0</version>
...