Hi - thanks for reporting.
The first issue about MONAILabelReviewer occurs for me too on linux, but it doesn’t have any impact on the MONAI Label module itself, so you can ignore that for now (although the packaging of that module should be fixed at some point). Could you file an issue to be sure the MONAI developers are aware of this?
Also the issue with the nightly was reported here and there’s a patch so I expect the preview will be fixed soon.
The source of your issue not having the Train and Stop buttons is that they only show up after you connect to a server. So you need to install and run MONAI Label server using the instructions here.
HTH