How to fix installation of Office 2021 on my PC

If it's telling you that 365 is still installed, then you want to double check that you have all of 365 uninstalled, not just some of it. If you are using Windows 11, press the Windows Key + I to open Settings, click Apps on the left panel, and select Installed apps. Look through the list and make sure you don't have a Microsoft entry that says "365".

David, did you get your Pro Plus license from third party?

The Pro Plus product is not designed for home users. It is a volume license, which means it is intended for organizations, not individuals. These licenses are managed by an organization’s IT department and are tied to a contract with Microsoft. Because of how volume licensing works, activation may depend on things like installation limits or periodic validation through an organization’s KMS server. Those processes require IT support from the organization that owns the license. That is why these licenses are not meant to be sold to individuals. Home users typically do not have access to the IT infrastructure needed to manage or maintain them. Unfortunately, some third-party sellers offer these licenses at very low prices. They may seem like a good deal, but they are often resold in ways that violate the licensing terms. Most buyers are not aware of these details until problems occur.

Such license is not simply installed, but rather, deployed. I am afraid you want to return it and ask for your money back to save you some headache down the road.