Hello @fkhan-mbl and thank you for reaching out to Community!
The PlantUML Viewer app is developed and supported by a third-party, so I would recommend reaching out to the vendor of that App, as there's likely some setting in the app that is preventing it to be correctly listed in the workspace's Marketplace.
You should be able to submit a support request to the developers of that app using this link.
Thank you, @fkhan-mbl !
Patrik S
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