Can't install Ruby on CodeBuild

Hi, It seems that whatever I do I can't get CodeBuild to build my Ruby project. I need Ruby version 2.7.4 so as always I added .ruby-version file in the root folder of my app with ruby-2.7.4 but the build fails with error:

rbenv: version `ruby-2.7.4' is not installed 

I set ruby-2.6 in my .ruby-version but the build fails again with error

rbenv: version `ruby-2.6' is not installed 

I tried installing proper Ruby version at build so I set this command:

pre_build:
    commands:
      - rbenv install 2.7.4

but I get the same error:

rbenv: version `ruby-2.6' is not installed

What am I doing wrong? What am I missing?