We have seen a few reports of this particular error on MacOs over the last week and we are investigating those reports to find a specific cause.
There can be a number of potential causes for the error mentioned and most of them come down to access restrictions/blocks/corruption.
Broken/corrupted admin permissions, firewall/security blocks on the system or network, corrupt client data, interference from other programs on the system, to using a Case Sensitive file format or drive encryption software like file vault.
So while we investigate on our end we are providing the following troubleshoot which has been reported by some to address the issue and allow a proper update/install.
Try doing an extended power cycle on your system. Turn off all devices including the modem. Wait 30 minutes, turn on the modem, wait 5 minutes, turn on any network devices, wait 2 minutes, turn on your computer and try the game.
You can try switching your DNS settings to use a free public DNS servers instead of your ISPs Servers:
We do not have individual steps for specific modems or routers so if you need assistance accessing or updating the devices, contact the device maker/provider.
In some cases there is a corrupt or misconfigured setting within the router or modem. We cannot say what that might be offhand but the easiest way to resolve these conflicts would be to factory reset the router and/or modem.
You may need to look up your specific model for the steps on how to do this and in some rare cases you may need to have this done by the ISP. Feel free to reach out to them for assistance if necessary.
Perform the steps listed on this article to remove the Blizzard Desktop Application from the system: Blizzard Support - Uninstalling the Battle.net Desktop App (For mac there will be some specific files listed that need to be manually removed)
Try running your system in selective startup mode: https://battle.net/support/article/200483
Our clients do not support installation on encrypted drives using programs such as File Vault. There can be issues installing, updating, launching or connection issues due to the encryption. If drive encryption has been enabled, please disable it and allow the drive to decrypt: Blizzard Support - Checking for FileVault
Create a new administrator account: Blizzard Support - Creating a New Administrator Account
Once the account is created, shut down the computer, wait 60 seconds, restart, log into the new profile and try the app/client.