can you run mpost from the prompt

Yes, that gives:
Sorry, I can't find that mem file; will try PLAIN.
I can't find the PLAIN mem file!

that's real weird

in the tex live cnf file I see

shell_escape_commands = \
bibtex,bibtex8,\
kpsewhich,\
makeindex,\
repstopdf,\

and for some reason mpost is not in there, can you check that?

(or set shell_escape = t instead of p)

I only find the following .cnf files:
    ConTeXt/tex/texmf-context/web2c/context.cnf
    ConTeXt/tex/texmf/web2c/texmf.cnf

Both do not contain shell_escape_commands.

They have:
    shell_escape            = t
    %openout_any            = a
    openout_any             = p
    openin_any              = a
    parse_first_line        = f
    allow_multiple_suffixes = f

Do I need to make a texlive.cnf file?