Coil 6 or plug 6 is bad. To rule out the COP, swap 5&6 see if the codes changes to a P0355, it if does, the COP is bad, if it stays P0356, the plug is bad. Or the connector to 6 is worn/no plugged in correctly. Look for chaffed wires on the connector.
2007 FX2 Supercrew Sport