For Arnold itself, you can ignore that message. Unless you have an Arnold license.
Couldn't register Arnold renderer in Maya PIT file (error while parsing .pit file). Please contact firstname.lastname@example.org.
For Maya, that message is a symptom of a problem with your Maya install: the Maya installation is not creating all the required files on your system.
If you finish the install (you can install without MtoA if you want), and then try to start Maya and nothing happens, then the ProductInformation.pit file is missing or corrupt.
The Maya install should create this file:
If there is any problem with that file, Maya (and any other Autodesk software) will not start.
Arnold will still render, but it won’t be able to use any Arnold license.
If that file does exist on your system, delete it.
Then remove all Autodesk software, and install Maya again (without MtoA)
You can install MtoA later (go to arnoldrenderer.com/arnold/download to get the latest MtoA).
If you still have problems with the Maya installation, log on to your Autodesk account to contact the Autodesk support team, or go to the Install and Licensing Autodesk forum at https://forums.autodesk.com/t5/installation-licensing/bd-p/24