I have a 2006 nissan altima 2.5 with a code reading P2101 (throttle actuator I think) the car will not crank. How can I fix this?

Make sure all the electrical connections to the Throttle Body are solid and there is no corrosion on the contacts.

You may need to replace your Throttle Body and/or the Throttle Position Sensor.

But first double check all the electrical connections.

Do you mean it cranks but will not run? Electric throttle control actuator consists of throttle control motor, throttle position sensor, etc.
The throttle control motor is operated by the ECM and it opens and closes the throttle valve.
The current opening angle of the throttle valve is detected by the throttle position sensor and it provides feedback
to the ECM to control the throttle control motor to make the throttle valve opening angle properly in
response to driving condition.When the computer demands a certain throttle opening it must see a result equal to what it demands or the code will set. That is one of many conditions that will cause a DTC. Open or shorted connections between the throttle components; high resistance {dirty connections}. It is absolutely necessary to have a quality scanner,an accurate wiring diagram, and a solid working knowledge of the function of the system and that is assuming the system is analog. I think it may be digital in which case in addition to all of the above you would need a dual trace oscilloscope,. Get my point..

Have a repair shop or dealer (if you wish to throw
away extra money for no reason),diagnose the code

You can Google the code,I won't explain it to you
for several reasons

Type: Nissan OBD PO2101 & then high lite your year
& read up

I have seen too many throttle bodies replaced based
on codes, when it wasn't the problem

So proper diagnostic work is required

You can have a: No Crank or a No Start --- Witch is it?

That code will not stop the starter from turning the motor over
(Cranking)

It should not stop the car from (Starting) Unless the ecu does
not see any throttle position or throttle plate angle data

Still should start,just not be able to use it very well

After all that ----------you know how to fix it-----------

You have used local shops & dealers your whole life to
work on your vehicles,so look at a site like this as a place
to ask for a guess of the unknown We have no vehicle to
test End of story There are dozens of reasons for any SYMPTOM
NOT what is the fix,that is the easy part

Finding A Problem That is the real issue ALWAYS