We'll need more information. Are you trying to create an email/landing page, or just store a file?
If you are just attempting to upload a file: Add Images and Files to Marketo - Marketo Docs - Product Documentation (max file size is 100mb)
If you are planning on uploading the html and looking to create an email / landing page from the html. You will need to create an email or landing page template: instructions as follows
Hope this helps