Cannot resolve org.springframework.boot:spring-boot-starter-web:unknown

This error occurs when Maven or Gradle is unable to find the required dependency in the repositories.

You can try the following steps to resolve this issue:

1. Check if you have added the correct dependency in your project's build file (pom.xml for Maven and build.gradle for Gradle).
2. Make sure you have added the correct repository to your build file. For example, if you are using Maven, you may need to add the following repository:

<repository>
  <id>spring-snapshots</id>
  <name>Spring Snapshots</name>
  <url>https://repo.spring.io/libs-snapshot</url>
  <snapshots>
    <enabled>true</enabled>
  </snapshots>
</repository>

3. Try refreshing your project in your IDE. For example, in Eclipse, you can right-click on your project and select "Maven" > "Update Project".
4. Check if you have an internet connection and that there are no firewall or proxy issues blocking Maven or Gradle from accessing the repository.
5. If the above steps do not work, try deleting the local repository cache (located in ~/.m2/repository for Maven and ~/.gradle/caches/modules-2 for Gradle) and then re-building your project.