What Flatpack lovers don't tell you is that you can have three small apps, like baobab, for example, each maintained by the developer in a different GNOME environment, so the first app is using "GNOME Application Platform version 40" as a repo, the second one is using "GNOME Application Platform version 41" and the third is using "...
If the Neovim application is not starting, then there is likely a missing dependency that needs to be installed on your computer.
Here is how you can find out the problem:
Open Terminal (if it's not already open)
Check your syslog file for errors:
sudo vi /var/log/syslog
Note: Feel free to use a different editor. The use of vi here is more muscle memory ...