The problem is, you don't have to Roblox Api Services enabled in studio.
To enabled this, go to the "Develop" tab on the Roblox Website, which was renamed to "Create".
Find your game and click the Gear on the right hand side of it, and press Configure Game from the drop down menu.
Check off "Enable Studio Access to API Services: [ ]"
Keep in mind with this disabled, it will save data to the actual server, so if you want to test saving data, I suggest disabling this option
The reason why you need this, is because without you don't have access to things like the "MarketPlaceService"
Correct me if I'm wrong, but I believe this will fix your error.
2 comments
Are you doing this in studio?
Yeah