Invoice Prefix should give error or filter out certain characters like /
The invoice prefix at civicrm/admin/setting/preferences/contribute allows characters like / to be saved in the Prefix name. For example "Organisation/2020/Invoices" saves without problems as Invoice Prefix.
But the character / is not allowed in directory names in Linux, see discussion https://stackoverflow.com/questions/1976007/what-characters-are-forbidden-in-windows-and-linux-directory-names
When trying to print an invoice there is an error in that case "Could not move custom file to custom upload directory".
So, or CiviCRM should replace the / by an allowed character like a dash (-) or should give an error message that the character is not allowed when saving the Invoice Prefix.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information