Adding index - There is insufficient system memory in resource pool 'default' to run this query
The problem is that the amount of memory available for memory-optimized data is not a fixed size. It depends on system needs. 

Please see my post here:
http://nedotter.com/archive/2018/01/in-memory-oltp-resources-part-4-oom-the-most-feared-acronym-in-all-of-in-memory-oltp/

Bottom line is that you need more memory.