In some of our EOS releases, you might run into the problem that the system cannot upgrade through App Center. You will able to follow this instruction to restore to the normal state. Please keep in mind, this is not a normal situation you are running into, upgrade failure only happens with following specific EOS release versions:
How to check your release:
To begin recovering from the situation, please check your Endless OS version before continuing to the following steps:
$ sudo ostree admin upgrade
After the command finishes its work, please reboot the system.
And now you should able to update your system through the App Center again.