unable to create partition on free space using disk utility
Both the Disk Utility application and diskutil command do not allow free space to be converted to a new partition. Instead, you are suppose to create new partitions by splitting an existing partition.

So first of all, enter the command given below to reclaim the free space to the existing APFS partition.

diskutil apfs resizecontainer disk0s2 0


Open the Disk Utility application and highlight the container disk1, then click on the Partition button. You should get an image similar to what is shown below. 



Next, click on the Partition button in the popup window. You should get an image similar to what is shown below.



Click on the +button, then enter a name. I chose Macintosh HD 2. Make sure the format is APFS and the size is 350 GB. The result should be an image similar to what is shown below.



Next, click on the Apply button. You should get an image similar to what is shown below.



Next, click on the Partition button in the popup window. The result should be an image similar to what is shown below.

 


  Note: The Disk Utility application may show incorrect results. You may need to quit and reopen the Disk Utility application to see the correct results.