Your report suggests one of two situations:
- You are trying to execute the regenerate_maildirsize script from the current directory, and your current directory is not /scripts. If so, please execute /scripts/regenerate_maildirsize
- Your current directory is /scripts but the file does not exist. If so, execute /scripts/upcp --force to force an update. Either you are running a very old version of cPanel or somehow deleted that script. The upcp will fix that.
If neither of those situations describe yours, please open a support ticket with your license provider so we can take a look at what is happening with your server.
ON the off-chance you are not the server administrator, or a reseller with the proper access to /scripts, then you need to contact your system admin to have that script executed.