If you had billing enabled for PlantUML for Confluence Cloud prior to the product becoming free, you may need to cancel the subscription manually. Firstly, go to the admin panel of the cloud instance:

Then "billing"

Then "manage subscriptions"

Pick "PlantUML for Confluence Cloud" pulldown menu and click DELETE. Note that we cannot add the plugin for billing to show it, we are showing another plugin to demonstrate, ensure you select the correct one.

Type DELETE. Note that no data is lost, the warning is the default shown by Atlassian and is incorrect.

Then go to the marketplace page for your Confluence instance, https://xxxxx.atlassian.net/wiki/plugins/servlet/upm/marketplace. Search for PlantUML and check whether it is installed. If it is installed, the process is complete, you do not have to do anything else. If it is not installed, install the plugin shown below:

Once installed the process is complete.