Hi there, if you are experiencing this message despite everything being up-to-date and the server not being full, please try these quick fixes!
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Run the launcher as admin; (Most common fix!)
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Make sure you’ve DOWNLOADED Discord, and joined our Discord server;
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Allow it as a trusted app in your antivirus options;
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Although unlikely, if your discord name contains special symbols/characters, you may need to change it to one without.
If the above solutions do not work, please write your Windows version below (press Win key > write winver > press enter)
If none of these worked, but you found your own method to get it working, sharing with the rest of the community would be greatly appreciated! 
Happy gaming!