DIAGNOSIS This isn't your Wi-Fi. It's a server-side error with Chamberlain's myQ cloud service. Their servers that handle user permissions and invitations are occasionally flaky or under maintenance.
FIX Don't just retry immediately. Force-close the myQ app completely on your phone. Then, unplug your myQ hub from power for 30 seconds and plug it back in. Wait for the status light to go solid green (online). Then reopen the app and try sending the guest invite again. This forces a fresh connection from your hub to their servers.
IF THAT DIDN'T WORK Try creating the guest access from a different device, like a tablet or a friend's phone on your Wi-Fi. Sometimes the issue is cached data in one particular app instance.
HEADS UP Guest access requires the primary myQ account holder (you) to have a verified email address on file. Double-check your account email in the app's settings and make sure you've clicked any verification link they sent when you signed up.