It's not possible to use the Repository Tag task in Bamboo Specs YAML

Workaround

On the YAML code, use an any-task configuration:

1. Configure the plan using the Bamboo UI
2. Export it to YAML Specs format
3. Observe the any-task configuration that was created for the Repository Tag task and make the necessary changes to the code

     - any-task:
         plugin-key: com.atlassian.bamboo.plugins.vcs:task.vcs.tagging
         configuration:
           selectedRepository: defaultRepository
           tagName: ${bamboo.buildResultKey}
         description: Add a tag
   

More information and examples:

• How to generate YAML Specs for any-task configurations