My employer is requiring 30 day notice before leaving or they will reduce my salary to minimum wage for the time worked.

My employer is requiring 30 day notice before leaving or they will reduce my salary to minimum wage for the time worked.

Is this in the USA? If not, then where?

Assuming you are in the US, they can not do that.

They can ask for a 30 day notice and give you a negative reference if you don't but commonly 30 day notices are not given in the US.
They may not reduce your pay for days already worked but can for any future days after they formally reduce your pay.