I’m stuck playing this mission don’t know how to buy the settlement it’s been a hr lol can someone help me
Welcome to the forum !
Let me help you finding the correct sub forum 
Currently you posted in age of empires 1 section, I will move it to the age4 section
I just completed this mission after have the same problem for three hours. :’(
It took me a while to realise the to buy the settlements you need to press RB when you are in the tribute menu to open the settlement purchase tab.