The error `java.lang.ClassNotFoundException: org.springframework.boot.SpringApplication` occurs because the Spring Boot dependency is missing or not properly configured in your project. To fix it, ensure that you have the following dependency in your `pom.xml` (Maven) or `build.gradle` (Gradle):

​

Maven:

```xml

<dependency>

    <groupId>org.springframework.boot</groupId>

    <artifactId>spring-boot-starter</artifactId>

</dependency>

```

​

Gradle:

```groovy

implementation 'org.springframework.boot:spring-boot-starter'

```

​

Then, refresh and rebuild your project.