Can't resize macOS partition (APFS error code 49187)
I solved this error for myself by the following steps, (to be sure I included all)


Time Machine snapshot delete fix
Decrypt harddrive from filevault


This resulted in the persistence of the error. But then I booted in recovery mode (reboot and hold cmd+R) and started the disk utility there. To my surprise the Disk Utility showed me different values for the free space of my partition. I only had 20g free instead of the 50g I tried to allocate each time. 

When I did the normal partition steps in the recovery boot Disk Utility everything went fine and I am currently running Ubuntu 18.04.1 alongside Mojave 10.14.1 without problems, be it that I only have 20g.