Unity Pro error: "No valid PLC project found"

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Problem:
This error is displayed in the output window when trying to open a Unity Pro project.

Environment:
Unity Pro

Reason:
The PLC type is either not supported by the Unity Pro package (e.g. opening a Quantum project in Unity Pro S) or the version of the
firmware called out in the project is later than those supported by the current Unity Pro installation.

Resolution:
Use the Unity Pro package that supports the PLC type that is configured in the project.
Open the project in a later version and drop down the version of the firmware configured for the CPU. Save the project as an .STA file
and reopen in the earlier version of Unity Pro.