I have enabled Prarallel comiplation in Quartus® Prime, but Analysis & synthesis reports that it is not working with multicore. Under what conditions does multicore work?

As described in the Quartus® Prime Standard Edition User Guide: 
If the partitions are separated by partitions, etc., they will be processed in parallel at compile time.

(Reference) Intel Quartus Prime Standard Edition User Guide: Design Compilation 
https://www.intel.com/content/www/us/en/programmable/documentation/pts1529446039343.html#mwh1410471195572
(Enabling Multi-Processor Compilation section)

(excerpt) 
For designs with partitions, once you partition your design and enable partial compilation, the Intel® Quartus® Prime software can use different This can cause higher peak memory usage during Analysis & Synthesis. This can cause higher peak memory usage during Analysis & Synthesis.

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Category:Quartus® Prime 
Tools:Quartus® Prime 
Devices:-