Project 'org.springframework.boot:spring-boot-starter-parent:2.4.0' not found

I just ran into a similar problem. I'm using IntelliJ IDEA Ultimate 2020.2. The version number 2.4.0 was red and the message was the same as yours.

Selecting File -> Invalidate Caches / Restart... and choosing the Invalidate and Restart option fixed the issue. I guess it was just a stale cache.