The message means just what it says, your computer doesn't have enough memory.
The best fix is to use a computer that has BOTH a 64-bit operating system and 8GB or more of memory. (8GB with a 32-bit operating system won't fix the problem)
In the short-term, you should restart your computer and make sure only Lightroom is running when you export and turn off as many background applications as you can (such as disconnect temporarily from the Internet and turn off firewall, antivirus, ... ).
What camera are the pictures from?
How big (pixel count) are your pictures?
Most likely the 32-bit version of LR is what is killing you.
Thanks for quick reply
I know, the new camera is the main problem is a D810 the file in Jpeg are around 36 mb. but this happens also with one file, some time I export 100 with no problems some time is impossible
The problem is most likely a combination of the 36 mp pictures from the D810 and the type of edits that you are doing.
I am guessing that the basic "whole picture" edits normally do not cause you problems while lots of spot/healing brushes or lots of adjustment brushes will cause problems.
You have LR 5.7.1. There is a 64-bit version of this -- you really want it -- but it requires you run 64-bit Windows.