On insider program settings check that you have Beta channel select and then try to check for updates.
What happen when you go to Settings and check for updates?
Do you see any new update or do you get error when trying to update?
If you can't select Beta channel then delete WindowsSelfHost key from registry,
Open regedit Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsSelfHost Right click WindowsSelfHost -key and select delete. Restart.
Restart and check insider program settings again and rejoin back to Beta channel.
If that doesn't help then download latest insider preview ISO build 26220.9586 for Beta channel. https://www.microsoft.com/en-us/software-download/windowsinsiderpreviewiso
Mount that ISO and run setup.exe from mounted image to start in-place update.