Handoff to Paypal Express with Craft Commerce
There is most likely some error in your configuration, or what you're sending to the cart.

The following code (when in devMode) will show you any errors that are coming back from Commerce/Paypal when you're redirected back to the cart:

   {# SHOW ANY ERRORS IF THERE ARE ANY! #}

{% if craft.config.devMode and (craft.session.hasFlash('commerce') or craft.session.hasFlash('notice') or craft.session.hasFlash('error')) %}

    {% set isErrors = true %}
    {% set commerceFlash = craft.session.getFlash('commerce')|join('|') %}
    {% set noticeFlash = craft.session.getFlash('notice')|join('|') %}
    {% set errorFlash = craft.session.getFlash('error')|join('|') %}

    Commerce: [{{  commerceFlash }}]
    Notice: [{{  noticeFlash }}]
    Error: [{{  errorFlash }}]

{% endif %}