If anyone else has/had this problem, here's what solved it for me. Thanks to the good folks at PreSonus. They sent me this info, which worked perfectly:
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When you upgrade from a previous version of Studio One Pro to Studio One 5, you don't get a new Melodyne license.
This means the version of Melodyne you previously registered from Studio One 4 must be installed and activated from your Celemony account.
You will not be able to install this from the Melodyne installer built in to Studio One 5. Uninstall any Melodyne that you installed from Studio One. Log into your Celemony account, download, install and activate from your Celemony account.
If you use the Melodyne from Studio One 5, it will not work.