Please check if you have a folder called 'Production' or 'Testing' in this directory:
home directory/documents/RealmofthemadgodTo get to the home directory use the keyboard shortcuts in the Finder: Command-Shift-H.
The issue might be caused if you have spaces or other special characters in the path to these files, specifically the <yourUser> part. Make sure that the user name you have in the path does not contain any special characters.
Next, if you have these folders - first make sure you have uninstalled any previous versions of the Exalt client and then delete both of the folders and try to install and patch the client again. Should that fail, delete them again (if they were even created this time) and try switching between 'Production' and 'Testing' environments in the Exalt Client before attempting to patching again.
If you don't have these, uninstall the game from Steam, try to download the game launcher from here - https://www.realmofthemadgod.com/ , create a temporary ROTMG account with an email address, if you don't have an email associated account already and then log into it. Once that is done you should be able to update the launcher and this will place the latest build in your documents folder. Now install the game from Steam again, and try starting the game from steam to see if that solves the issue.
Alternatively, you can remove the Production and/or Testing folders, uninstall the game, create a new user account with full privileges on this computer and install Exalt again on that user to see if you can download the patch and play the game.
If that works, then you likely need to rename the user folder for the original user account.
Make sure you rename the original user account folder to something without spaces and/or special characters. Only use A to Z.
Please note that if you do not feel comfortable fiddling with the settings in order to fix this, then the other workaround remains to play ROTMG from the other user account or to try and copy the build folder from the new user account to the original one manually.