I have the same error on the secret variable definition. I make the mistake of indenting incorrectly as if they depended on the services, when they don't.

services:
 # ...
  secrets:
    db_secret:
      file: .env.local

to fix it

services:
 # ...
secrets:
  db_secret:
    file: .env.local

An error as big as the missing semicolon..