2002 Toyota Highlander squealing sound

Typically squealing from the belt is usually lack of tension on the belt or in other words tightness. This could also be the belt itself if it is worn out. Spraying water from a squirt bottle onto the belt while the engine it running will usually make it quieter for a while. If it does then the noise is coming from the belt whether belt itself or the lack of tension. The belt has an automatic tensioner pulley which automatically adjusts the tension through a pulley. I would suspect the belt and/or tensioner before the a/c compressor.
If your mechanically inclined: removed the belt and check each pulley that they spin freely except for the crankshaft pulley

I hd the belt replaced and sprayed it many times. The gargage tells me it isn't the belt. I am a woman so I know nothing about cars. I wanted them to adjust my belt but they won't. Now my check engine light came on and they say it is from the ac compressor. What do I know they want do do this job and I am not convinced that is the problem.

There is no way of adjusting the belt tension manually because the tensioner pulley automatically does it.
Spraying the belt with water is for diagnostic purposes only. If the squealing noise goes away briefly with the water then it is the belt making the noise.
As for the check engine light.
Most auto parts suppliers will scan your vehicle for free. Give the code(s) to me so I can assist you. Usually the compressor will not set a check engine light
If possible I would suggest a second option on your vehicle.
For squealing noises it is usually the belt, belt tension, or one of the pullies the belt rides on.