I am trying to place components in Altium designer using the Manufacturer part search but the "Place" option is inactive.
Not all components in the manufacturer part search can be placed directly into the design. The green circuit symbol shows if the required model data is available. You can directly place components with this symbol. For other components, you might have to create your own footprint and schematic symbol or use other external libraries. You can still use the parameters that are provided in the manufacturer part search though. Also notice that you can specifically filter for "Has Model".
Source: Altium Documentation