If the Agent fails to connect due to an authorization error, it could be that the certficate has expired. This can happen if the Agent has been offline for a long period of time. If this happens you will need to reinstall the agent by clicking Add Agent on the Agents page and following the instructions.
If this happens after installing the Agent using the install command you copied from Add Agent, it could instead be that the one-time token included in the install command has expired. These tokens are short-lived and can only be used for one Agent each, so in this situation you'll need to click Add Agent again and copy and run the new command.