Could you perhaps elaborate a bit more? What period are we talking here, Spanish galleons covers a wide range of ships and an even larger time period. Then there's the country and capital in question, is it a fortified capital? Would cannon and defences already be in position for your protagonist? If no then they may not be able to stop them from taking the capital directly, let alone the rest of the country. Coastal defences don't just appear overnight.

Without considering that, the answer is most likely no. Without a fleet to oppose them at sea and with however many miles of coastline completely unguarded you will not be stopping them from landing. Armies were quite capable of deploying themselves without use of a port, from ages like William the Conqueror's invasion of England in 1066 to the British Navy landing men and material in the Netherlands and Denmark during the Napoleonic wars.

https://en.wikipedia.org/wiki/Spanish_Armada#Planned_invasion_of_England

The Spanish Armada in 1588, for example, planned to land the Spanish army outside of London, not attack London itself. This shows a clear ability to deploy all the necessary men and material to maintain an invading army without the use of a port.