Unfortunately, one of the quests in the "Home island" section cannot be completed.
The quest asks you to find all available deposits on the island with your geologists (stone, copper, marble, coal, iron and gold), and sets a target of 55 deposits to find.
Sadly, it is only possible to score 54/55.
Either the system is counting the two marble deposits in Sector 1 where you can only have one or the other, or: it is counting the "phantom" marble deposit known to lurk around Sector 8 .
Whichever is the case, this quest cannot be completed. Back to the bug server please.