There are several reasons that may cause your PayPal Express to not work:

1.Currency

If your default currency is not USD (the default currency in PayPal), you need to confirm that your currency is accepted by Paypal. Also, make sure the currency code in your dashboard ("Settings"->"Localisation"->"Currencies"->Code) is in accordance with Paypal.

2. Digital download products

PayPal Express is not supported for digital download products. If you sell digital download items you can select other payment options.

3. Own domain name.

URL forwarding and iFraming for your own domain are not allowed by Paypal Express because it is regarded as insecure. You may need to set up your own domain via theĀ 
standard steps.

4.Products that don't require shipping

Only products that require shipping can checkout with PayPal Express, because PayPal Express only supports products that have a shipping rate.