V speeds don't show on the PFD speed tape and VNAV won't arm or engage

PMDG 737NGX
This is most likely caused by failure to execute the performance initialization on the PERF INIT page.  The white light will be lit after you finish filling out the required fields and you must press the EXEC key for it to "take" the data.  After that, the V speeds should show and you'll be able to arm VNAV.