JS trying to define a variable inside a class type obj TypeError: Attempted to assign to readonly property

This should be quick, I have identified the issue but don’t know how to fix it, here’s the code:

class Character {
         constructor(min, max, name, lastname){
                this.name = name;
                this.lastname = lastname;

                this.x = getRandomArbitrary(min, max);      
                this.incert = 0.167*Math.E**(0.0109*this.x)
                this.AveParam1 = -37.973*Math.log(this.x)+259.73;
                this.AveParam1 = this.AveParam1.toFixed(2)
                this.AveParam2 = 0.6904*Math.log(this.x)-2.5279;
                this.AveParam3 = 12.197*Math.log(this.x)-57.259;
                this.AveParam4 = 12.658*Math.log(this.x)-59.645;
                this.AveParam5 = 12.428*Math.log(this.x)-40.302;
                let y = this.incert * (-1)
        
                this.Param2 = this.AveParam2 + getRandomArbitrary(y, this.incert);      
...

It seems like the issue is in the last line, how I define this.Param2 seems to be the issue. I tried removing getRandomArbitrary(y, this.incert) but that doesn’t fix it. Maybe I should be using a keyword I’m not aware of? honestly don’t know what’s wrong, I’m self taught.