PDK ships with the puppet debugger by default in the form of a new pdk console command. You can view the RFC here for the console command.

If you wish to keep a more updated version of the puppet-debugger gem or pin to a specific version you can use the following syntax in your .sync.yml file. This is not required as the debugger ships with PDK 1.14+

# .sync.yml
      - gem: "puppet-debugger"
        version: "~> 0.19"

pdk update
pdk bundle install
pdk bundle exec puppet debugger

Example invocation

  puppetlabs-stdlib  pdk console
Ruby Version: 2.5.7
Puppet Version: 6.13.0
Puppet Debugger Version: 0.19.0
Created by: NWOps <corey@nwops.io>
Type "commands" for a list of debugger commands
or "help" to show the help screen.