macOS 10.15 and later versions won't usually allow you to install things into folders at the top-level of the root drive, such as the folder in your error message (/Hauptwerk).
Hauptwerk v4 and earlier installed to /Hauptwerk by default, but for Hauptwerk v5+, for compatibility with macOS 10.15+ the default (for a clean installation) was changed to:
[your-macOS-account-home-folder]/Hauptwerk
... which should be fine.
If it's still defaulting to /Hauptwerk then presumably you used macOS' installation wizard to transfer settings/applications to the Mac from another Mac which previously had v4 installed on it when you set up your new M1 Mac, resulting in the v6 installer picking up your previous v4 installation paths and defaulting to them (as it would for an upgrade).
Please try following this step from the 'upgrading' section from the Hauptwerk user guide (page 26 in the current v6.0.2 version):
Step 6. Very important: If upgrading from a pre-5 Hauptwerk version, use Finder to make sure that all of the following folders are completely deleted, if they exist (you may be prompted for your password):
1. /Applications/Hauptwerk Virtual Pipe Organ
2. /Users/Shared/Relocated Items/Hauptwerk
3. /Hauptwerk
(The latter folder is at the very top-level of your computer's SSD/hard-drive. In order to be able to find it, make sure that the Sidebar | Devices | Hard disks option is ticked in Finder's preferences.)
... then reboot for good measure, then do a clean install of v6 and leave the installer's folders are their defaults (or if you want to customise them then make sure you don't select any folders at the root level on your Mac's system drive).
Does that solve it?