Can't partition SDD with odd error
In order to change the size of a partition, the partition can't have any errors.

The easiest way to repair the partition is to reboot while holding Cmd+R, and start the Disk Utility from the menu that is presented after the recovery system has started.

Then select the partition and click the 'Repair Disk' button. After it has finished, it should say that there are no more errors. The partition can now be resized.

NOTE: Unfortunately, because of HFS+ it is possible that Disk Utility may display "Could not be repaired", at which point you would need to reformat the drive and restore from your latest backup. I hope this is not the case!