Why am I getting Unknown error in line 1 of pom.xml?

answer according to current status

This issue got fixed:

Please install the m2e connector for mavenarchiver plugin 0.17.3 from https://download.eclipse.org/m2e-wtp/releases/1.4/

obsolete answer

A less profound change than a downgrade from Spring Boot 2.1.5.RELEASE to 2.1.4.RELEASE would be downgrading only the affected Maven JAR Plugin from 3.1.2 to 3.1.1 as long as this bug exists:

<properties>
    <!-- ... -->
    <maven-jar-plugin.version>3.1.1</maven-jar-plugin.version>
</properties>

Add 3.1.1 in to properties like below than fix issue

<properties>
        <java.version>1.8</java.version>
        <maven-jar-plugin.version>3.1.1</maven-jar-plugin.version>
</properties>

Just Update Project => right click => Maven=> Update Project

For me I changed in the parent tag of the pom.xml and it solved it
change 2.1.5 to 2.1.4 then Maven-> Update Project

As per the suggestion from @Shravani, in my pom.xml file, I changed my version number in the area from this:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.5.RELEASE</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

to this:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.4.RELEASE</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

I then right clicked on the project and did a ‘Maven -> Update project…’. This made the problem go away for me.