I have a 97 previa the transmission is not engaging at all

No one any web can answer that. (GOD like , huh?)
but the car , IIRC has a TCM operated transmission
why not get the TCM scan codes.
the FSM tells you how to do that.
is the transmission ground strap missing or gone or bad?
if yes, the TCM is now useless. fix the ground first
or add one.
the correct question is answered in the FSM.
A./T are complex and only using the FSM will you get correct answers

1997 can be a transistion year.
IN USA< why not state country, so we can stop the wrong answsers.....
the 1997 car many have the TCM integrated into the PCM
in fact the word PCM means just that.
and any USA scan tool on your USA car willl scan
and you will see things like P07xx , tranny codes
if the TCM is seperate (too lazy now to look at alldata.com)
the Old TCM on all cars (most) you place jumper wire and the silly thing blinks out DTC 2 digit fail codes.

once this far and no errors.
the FSM says.
do the stall test. yours fails. 100% (and my ground check)
then you do the pump pressure tests..
also some TCM you can put the TCM in to manual mode.
and shift car by hand, at the shifter.
that too is in the FSM. for most cars.
lots of on car tests.