This feature has to be enabled by Canvas for your instance
This option has to be enabled by Canvas. Here is an excerpt from the Canvas support doc titled, "How do I upload custom JavaScript and CSS files to an account?"
"Before adding custom JavaScript and CSS files, you must contact your Customer Success Manager to enable custom branding for your account and any sub-accounts. Once custom branding is enabled, the option to upload custom CSS and JS files is available in the account and all sub-accounts."
Because DesignPLUS requires custom code to be added to your Canvas theme, it cannot be installed in your Canvas instance until the option to upload custom CSS and JS files is enabled. Please reach out to your CSM at Canvas to enable this feature. They can usually get it done fairly quickly (about 1 - 24 hours).