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. More information