When the newly added partition on your Mac primary drive can not be mounted, you can retry to force mount it again in Disk Utility or use Terminal command.
Before macOS Catalina, you can only choose to partition the container on your Mac device. The encrypted drive should be automatically mounted when you check it in Disk Utility. If not, you can select the partition and click the Mount button in Disk Utility's toolbar, or open Terminal and run the command diskutil mount /dev/disk1s2(replace the drive identifier with yours) to force mount the drive.
If you partition the Mac primary drive and encrypt it for data security, you can also choose password-protect files or folders on Mac to prevent other Mac users from accessing. In this way, you don't need to partition the drive. On macOS Catalina and later, you can choose add an encrypted-APFS volume to the container instead of partitioning, the APFS volumes share the free available space within the container.
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