I installed ola and olc on Lubuntu according to the directions in README.md
Version displayed in the About window: 0.6.beta
I do not know how to use it.
All the tabs are blank, except a few tabs have some table headings, such as Sequences, independents, Master List and Main Playback.
The Channels tab is empty.
Virtual Console is a blank window with a single digit 1 displayed.

msgid "Rotatives Mode" should be "Rotation Mode"
msgid "+% et -% value" should be "+% and -% value"

msgid "Sequence +" should be "Next Sequence"
msgid "Sequence -" should be "Previous Sequence" or "Prev Sequence"

I can see the following log coming from OLA while running the flatpak:

common/io/IOUtils.cpp:39: open(/dev/dmx0): No such file or directory
plugins/opendmx/OpenDmxPlugin.cpp:80: Could not open /dev/dmx0 No such file or directory
common/io/IOUtils.cpp:39: open(/dev/kldmx0): No such file or directory
plugins/karate/KaratePlugin.cpp:79: Could not open /dev/kldmx0 No such file or directory
Traceback (most recent call last):
  File "/app/lib/python3.9/site-packages/olc/application.py", line 271, in on_fd_read
    self.ola_thread.ola_client.SocketReady()
  File "/app/lib/python3.9/site-packages/ola/OlaClient.py", line 803, in SocketReady
    self._channel.SocketReady()
  File "/app/lib/python3.9/site-packages/ola/rpc/StreamRpcChannel.py", line 88, in SocketReady
    self._ProcessIncomingData()
  File "/app/lib/python3.9/site-packages/ola/rpc/StreamRpcChannel.py", line 269, in _ProcessIncomingData
    self._HandleNewMessage(data)
  File "/app/lib/python3.9/site-packages/ola/rpc/StreamRpcChannel.py", line 283, in _HandleNewMessage
    self.MESSAGE_HANDLERS[message.type](self, message)
  File "/app/lib/python3.9/site-packages/ola/rpc/StreamRpcChannel.py", line 327, in _HandleResponse
    self._InvokeCallback(response)
  File "/app/lib/python3.9/site-packages/ola/rpc/StreamRpcChannel.py", line 370, in _InvokeCallback
    response.callback(response.controller, response.reply)
  File "/app/lib/python3.9/site-packages/ola/OlaClient.py", line 932, in <lambda>
    lambda x, y: self._GetDmxComplete(callback, x, y))
  File "/app/lib/python3.9/site-packages/ola/OlaClient.py", line 1481, in _GetDmxComplete
    data.fromstring(response.data)
AttributeError: 'array.array' object has no attribute 'fromstring'

Not sure if they break some functionality.

The bit about missing devices under /dev/* can be ignored I think OpenLightingProject/ola#513. Maybe those plugins should be disabled.

OLC doesn't seem to use GtkFileChooserNative that supports portal API but GtkFileChooser

Portals allow to dynamically grant access to users rather than hard-coding arbitrary filesystem access for example for an application. The primary benefit here is tighter sandboxing; also better desktop integration.

Following instructions in README on github:

$ sudo ninja -C builddir install
ninja: Entering directory `builddir'
[0/1] Installing files.
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/mesonbuild/mesonmain.py", line 146, in run
return options.run_func(options)
File "/usr/lib/python3/dist-packages/mesonbuild/minstall.py", line 811, in run
installer.do_install(datafilename)
File "/usr/lib/python3/dist-packages/mesonbuild/minstall.py", line 564, in do_install
d = self.check_installdata(pickle.load(ifile))
AttributeError: Can't get attribute 'RunScript' on <module 'mesonbuild.build' from '/usr/lib/python3/dist-packages/mesonbuild/build.py'>

ERROR: Unhandled python exception

This is a Meson bug and should be reported!

FAILED: meson-install
/usr/bin/meson install --no-rebuild
ninja: build stopped: subcommand failed.