That is probably the end of the DEMO version, you need to check all of your current quests, if they show a "END OF DEMO" label then that's it.
Viewing post in Z-Island UnrealEngine5 Demo comments
It's not, there 2 versions of ZIsland the UnrealEngine 4 version which is available here: https://gamingtalesstudio.itch.io/z-island
And an UnrealEngine 5 version (DEMO) which will contain more side quests content but less characters here: https://gamingtalesstudio.itch.io/z-island-demo =)