why does PayPal say this transaction cannot be completed

What does mean "This transaction cannot be completed" in PayPal?

This transaction cannot be completed in PayPal because Sometimes payment or transaction is declined due to incorrect sender details, bank server error and some other PayPal terms & conditions. You get PayPal error code 10417, PayPal error code 50027, PayPal error code 50023, PayPal error code 15005, PayPal error code 10486, PayPal error code 12, PayPal error code 10002, PayPal error code 10002 security header is not valid, PayPal error code 10201, PayPal error code 10626 and PayPal error code 10001.

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