I faced the same issue in Eclipse Oxygen 4.7.1.
When I created the project for the first time, AEM plugin actually created the project on the file system but it gave the error
"Unable to create project from archetype [com.adobe.granite.archetypes:aem-project-archetype:11 -> https://repo.adobe.com/nexus/content/groups/public]"
I went to the workspace and i can see the project structure created there. Since the project creation was successful, Eclipse was not able to create the project with the same name. I found the error details in Eclipse log file located at <<Eclipse-Workspace>>/.metadata/.plugins/org.eclipse.ui.workbench/log.
!MESSAGE Unable to create project from archetype [com.adobe.granite.archetypes:aem-project-archetype:11 -> https://repo.adobe.com/nexus/content/groups/public]
!STACK 0
org.apache.maven.archetype.exception.ProjectDirectoryExists: A Maven 2 project already exists in the directory