Google Play uses an upload key to verify that new versions of your Android app are coming from you and not from someone impersonating your account. If that key is lost, corrupted, or was never generated in the first place, you won't be able to push updates until it's replaced — so this is worth handling as soon as you notice the problem, not right before your next release.
Start in the Google Play Console for the app in question. Under App Integrity, look for the option to request an upload key reset. Google will ask you to confirm your identity as the account owner and may require a short waiting period before the reset takes effect — plan for that delay if you're working against an event date.
Once Google approves the reset, you'll need to generate a new key and share the public certificate with your Amego contact so future builds are signed correctly on our end. Don't send us the private key itself — only the certificate. If you're not sure which file is which, ask before you send anything; re-issuing a key is straightforward, but mixing up the private and public halves creates a mess that takes longer to untangle.
After the new key is in place, request a test build before your next real release. It's a lot easier to catch a signing mismatch on a test build than to discover it when you're trying to ship an update the week of your event.