I am new to Picrust and trying to run the example data set following the commands on https://github.com/picrust/picrust2/wiki/Swapping-in-SEPP-for-read-placement-with-q2-picrust2, but when I run the command - " qiime fragment-insertion sepp --i-representative-sequences mammal_seqs.qza --i-reference-database picrust2_default_sepp_ref.qza --output-dir tutorial_placed_out --p-threads 1 "
Note: I have changed the title since this is really a SEPP error, and an error you are getting from QIIME 2, not from PICRUST itself, per se.
I am pinging @gmdouglas since these are tutorial files causing this error, in case he wants to take a look.
Given that these are tutorial files, I agree with you that this is probably a memory error, but you can follow the debugging steps in this forum topic (note the debug and verbose flags) to get more diagnostic details: