Please check the read & Write permission for the disk.
I have rights for this disk. I can create new folder on ramdisk in Choose folder dialog in lightroom, so technically, lightroom/user has rights to write to ramdisk. And I get this error already when export folder choose.
next informations: windows 8.1, LR 6.4.
Why on earth are you trying to export to a Ram Disk. Your problem is probably because you don't have enough available space on the RAM made to look like a physical hard drive.
It's my thing, and it's not relevant. Error is "not writable" and not "not enough available space". btw it's possible to export to usb stick with much less available/total space then ramdisk.