When try to setup webmail package ppa_webmail on management node you have got error:
ppa_agent 1.0.13 (other) Conflict with package: PleskIntegration 18.104.22.168 (sc) ppa_core 1.0.72 (other) Conflict with package: ppa_agent 1.0.13 (other)
PPA management node installation is supported only on 64-bit architecture systems.
Management node automatically contain installed webmail package.
If you are trying to install package manually via PPA web interface, you will be able to select only 32bit package, which is not compatible with management node.
It is not necessary to install webmail package on PPA management node manually, because it is installed by default. You can read more about services on management node in Parallels Plesk Automation Deployment Guide, on page 5 "Hosting Services Provided by the Management Node".