It's likely that a technical problem with the Facebook platform is the cause of the error you see while attempting to open your event manager in the Facebook Ads Manager. An issue with your browser or internet connection might also bring it on.
Here are some troubleshooting steps you can try:
Clear your browser cookies and cache and try accessing the Event Manager.
If the issue still persists, try to access Event Manager from a different browser or device to check whether the issue is specific to one browser or device.
Ensure you are logged in the with correct Fb account with access to Event Manager.
See if there are any known issues or outages with their Facebook platform by checking the Facebook Help Center or visiting their Twitter account.
If none of these steps solve the issue, you may need to contact Facebook support for further assistance.