Artemis to Level 15?

How? I spent seasonal coin getting all the buildings required to complete city albums in order to get the extra seasonal coin. 
I’m almost done Mayors pass and there are no more seasonal coin rewards. There’s 8k to collect on the other reward   I’m at Level 9. That will take me maybe to Level 11 (can’t see as it dues t show you the next requirement)

How are we supposed to get to Level 15 to complete the final City Album to get the final building.