Currently, use of ObjectBox from a sandboxed macOS application requires you to set up at least one App Group for ObjectBox to be able to open a database, in addition to whatever you would normally have done to gain access to the destination folder under the sandbox:
Open your project in Xcode.
Select the project icon at the very top of the Project Navigator.
Select the Signing & Capabilities tab where you turned on App Sandbox.
Click the + Capability button and double-click App Groups.
If the App Groups list is empty, click the little "+" icon to add one.
You should now see a list entry like
FGDTDLOBXDJ.$(TeamIdentifierPrefix). If you only see
$(TeamIdentifierPrefix) , you still need to set up your team and code-signing under General.
That should be it: ObjectBox will pick up the App Group Identifier without any additional work.