"APFS Container Resize error code is 49153" when adding MS-DOS (FAT) partition
Try this workaround:


Instead of creating a FAT partition, create a APFS partition of the desired size with Disk Utility.
Select the APFS container that contains the new partition and click Erase on the toolbar:


Select MS-DOS (FAT) from the pull down menu, correct the name of the partition as necessary and press Erase:



This will replace the APFS container with a FAT partition.