In case the country of your card issuer is in the US, then you cannot use Amex card to pay via Skrill. The reason is that Amex card (American Express Cards) is restricted in the USA. And due to licensing issues, Skrill currently do not support Amex payments for US customers.
There is also other reason that you cannot use Amex to pay via Skrill. Most of the failed payments from Magestore are rejected by Skrill's fraud engine due to mismatch between the country of the registration of the Skrill Account and/or the country of the login/payment instrument issuer. As Skrill considers such mismatches as risky, they block the respective transaction attempts. To increase the chance of successful purchase, we suggest not to use proxy servers or VPN connections.
About payment method via Paypal, you still can use Amex card to pay via Paypal account. In case there is any problem during the checkout process, please contact your bank and place an order again in our store.