This error can occur due to insufficient permissions set on your account.
Go to the Gear Symbol > Manage Account, then click Campaign Managers:
Then find your account and click details:
Next click on one of your roles under the custom roles area:
Now check the box next to "Marker for Salsa Enterprise Clients" and click "save package permissions":
NOTE: If you do not see this package option, then the package may need to be installed!
Lastly, log out of Salsa, then log back in and you should be able to upload the file. (You may also have to clear your browser's cookies and cache in order for these permissions to take effect, but just try logging out/in first.)