Why doesn't the ac compressor clutch disengage?

Its either a mechanical or electrical problem. Check ac clutch fuse then check for voltage at the connector to the clutch. Next check continuity through the coil in the ac clutch. Some clutch actuators are brushless, older models might have a carbon brush that can wear out. The shop manual will tell you which you have. If all the electrical tests check out the clutch is seized mechanically. If the clutch cannot be separated from the pump the ac system will need to be discharged safely before the pump assembly is changed. Then re-gassed properly. You might be able to make a deal with your local garage or tyre fitting bay to help you with this.