Hello,
Thank you for making a bug report!
Generally this occurs when one of a few things happen. Either the Server is offline, you can check the server status in the discord server here: Discord
Or, when your client is out of date. You can update your client by going to the installation folder for YGO Omega, by default it is C:\YGO Omega, and running the OmegaUpdater.exe.
This will force your client to check for updates and force an update if needed. This can take a few attempts to get all the updates needed, depending on how long it has been since you have last used Omega.
If you have any other questions, please feel free to reach out in the #tech_support channel of the Duelist Unite discord server.
If you have not already joined, the invite link to the server is here: https://discord.gg/WrE2gMPS
Please mark this as the solution so that this thread will automatically close in 24 hours after doing so.
Thank you, and we hope you are able to enjoy Omega!