elementary OS 5 Juno and later does not have a “system tray” or support app indicators, as this is a decade-deprecated API and has been replaced with more useful, purposeful APIs like CloudProviders. You can read about all of that in this elementary blog post.
Since Dropbox is a proprietary app, only Dropbox themselves can update their app to support modern ...
After some test in my Elementary OS 5.1 maybe you need have this packages
For the line
(pcloud:3274): libappindicator-WARNING **: 19:22:30.549: Unable to get
the session bus: Unknown or unsupported transport “disabled” for
$ dpkg -s libappindicator1
Status: install ok installed