Please refer this thread Not Enough Memory Error
4gb might not be enough when you are running other Apps.
Please refer System requirements for Photoshop Lightroom for Mac and Windows OS.Recommended memory is 8 gb,
Try running Lightroom when no other Program is running,
4GBs of RAM is the bottom amount. I suggest you upgrade the memory to 8GBs. But then exactly how much free space is left on the hard drive (SSD if one is installed).
That message can mean 2 things. Not enough RAM available or not enough hard drive space available.
Also check that the folder you have selected for the export to be saved to has write permissions set for your Username.
I'm working on it and i'll send feedback for you to know how i solved the problem.
You've touched a little point i didn't mention:
the pictures i'm working on are stored in a server connceted to my pc. They are not in my hard drive. And that server has only 70 mb of free space on a total of 400 Gb!
But i've also tried to work on some pictures stored on my desktop and the error occured! My computer has no problems os free space.
The permissions you've mentioned are something that i have to talk about with my system administrator.
Later i'll tell you something. Meanwile thanks for your help!