Tightening steering rack

What do you mean by "tightening steering rack"?
If it's loose, the tig rod ends are worn out. Maybe just the outer ones. But maybe the inner ones
are worn also.
I believe what you have circled is just a cover.

 

It is a cover, and there is probably an adjustment behind it, and one that is made very carefully at the factory and sealed in. I am pretty sure I remember reading a document from Toyota saying how very much they discourage disturbing that adjustment (as in, if you discover that you've disturbed it, just buy a new rack), but at the moment I'm not remembering where I saw it.

 

I have removed and adjusted that exact adjustment on Corolla Karina and other Toyotas I have not on a Prius electric rack but that adjust the round part of the rack that slides side to side that will gently tighten up the slop that made develop over a lot of miles I'm talking you're moving that adjustment My god a 16th of an inch maybe it's like an adjustable shim it's kind of circular on the other side you can't see and it pushes against the round part of the rack and keeps it from being able to rock more or less for and a half or an excessively the rack might try to roll around but that's not possible but it would make a horrid noise so be very careful with that adjustment if it is even in fact able to be made on the electric rack

 

If it makes any difference, only the Gen 1 Prius had an "electric rack" (the torque sensor and motor were built into it).

Later generations have the torque sensor and motor drive as parts of the steering column under the dash. The rack is more conventional.

 

Then it probably has that large 22 24 mm hex adjustment circled in the red pic. I would think but I haven't looked at the back of my Gen 3 to see if it's got that adjustment we haven't had any trouble with the Gen 3 steering so far in any of them That's one thing that seems to work very well maybe until you wreck the car or something.

 

It is possible on your Prius that hex doesn't do anything it may be just an indention in the casting it may not be a true adjustable setting fitting I can't see it from here and I don't have mine up in the air to look at but on the older cars they had a wrench that dropped in that hole out of 22 mm 22 and 1/2 whatever it is and you would just twist that back and that plug would come out and the piece behind it was the exact opposite of the piece that it butts up against and there might even be a spring in there or something look on your rack breakdown if you have one