What’s happening?
Multi-select lookup control is not available for model driven apps except Dynamics 365 Field Service.
Reason:
The Multi Select Lookup Control feature is exclusively available for environments where Dynamics 365 Field Service is enabled.
Resolution:
As a workaround, a custom PCF (Power Apps Component Framework) control to achieve similar functionality. Information on creating custom components can be found at Create and build a code component - Power Apps | Microsoft Learn.
Alternatively, prebuilt multi-select controls may also be found from various sources online.