No pool was specified, although a pool is specified in the Azure pipeline

I have a problem in the ADOS system that the pipeline fails because there is no "pool" specified. Also the validation shows this error. However, I have defined a pool.

trigger: 
  branches:
    include:
      - 'main'

pool: 
  name: my-pool
  demands:
    - my_pool_demands

[...]

Do you have any clue?

I tried to

• run the pipeline with other pool
• run the pipeline without pool
• run the pipeline with minimum tasks (only build task)
• run the pipeline without any comments in it

Nothing could change the "No pool was specified" error.

After hours of modifying my pipeline and reading the documentation, I finally figured out how to fix this problem. It is not because of the yaml file. The error must be related to the pipeline. The solution is to create a new pipeline. The same yaml file will work fine in the new pipeline.