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I am having trouble with my vehicle. It slows down while I am driving and the warning comes up Reduced Engine Power. What can be causing this?

That is extremely vague. No vehicle information, make model, year, motor.  
 
I've had this happen to me recently though as i have a turbo charged motor and there was an ice blockage inside the inter cooler. This can also be related to the turbo its self having a fault.  
 
With no information that's all the info I can provide, my experience with a limp mode.

I have a 2008 Buick Allure 3.8

There's many expensive reasons for power failure. Stretched timing chain/belt, failing turbo (if there), head gasket problems, ignition problems and the list goes on. However, try the easiest and cheapest fix is to go out and buy and change out the air filter. Anyone can do that on their own. Another, problem might be fuel. That's more complicated because there's more places that problems crop up. Blocked injectors, clogged fuel filters, failing pumps, electronic quirks and the list continues on... But if I was in your situation, I would grab an fresh air filter and a bottle of brand named fuel injector cleaner to put in the car. Then just take it for a good long drive and see if that does anything.  
 
Just my opinion, I could be wrong.

it to a mechanic and have him run a diagnostic check, the codes should give a more precise reason for your problem.

Issue on an F150-turned out to be a computer problem. I had to change the ECM-took forever to diagnose because the problem was on again off again. Until it died completely and stayed dead they couldn't get a computer readout on the problem.

I have the same motor, pretty familiar…  
There are two reduced power modes, one caps the speed limit at 100, the other doesn’t let the motor rev over 2500 rpm.  
Which?

Thank you. I will.do that tomorrow.

I have no idea. I was driving on Victoria Avenue and every red light when I would step in the gas it would act like it was in neutral and give the warning then go away til the next red light. Was good after I shut it off and restarted.

You need to get the fault codes read with a scanner or preferably have a professional technician diagnose the issue or you will have no idea what to do. By the sounds of it this is already creating a safety issue in traffic and needs to be addressed and fixed properly.

throttle position or pedal sensor for speed restriction.

throttle actuator control circuits, throttle actuator command vs actual position fault, return spring check fault, or a throttle position circuit fault If rpm restricted

Sorry, this website kept giving me errors with my response so I had to cut it up. The first reasons are if your max speed is capped. The second of the rpm is capped. Those are the conditions that trigger that light.  
 
You need a scan tool that can pull live data to diagnose what it is  
 
Edited by Flele, 2021-06-02 21:11:39

....I got a mechanic to look and an oxygen oxygen sensor is unplugged for whatever reason....which really doesn't explain why the rooms are restricted all of a sudden if it was unplugged for how long....

Wouldn’t think a fuel mixture issue would set that reduced power situation but whatever… if you’re rolling now all is good

That is actually the entire purpose of an oxygen sensor. It measures the amount of oxygen in the exhaust, both before and after the catalyst converter, and keeps adjusting the fuel mixture ratio to get the optimum mixture. If its unplugged or can't sense any oxygen, it will restrict the fuel.