Change:

    If(AccredList.size()>0)
    
        return 'Opportunity Number is ' + AccredList; 

To something like:

    If(AccredList.size()>0){
    
        return 'Opportunity Number is ' + AccredList; 
    }
    
    return 'No Opportunity Found'; //or similar

The way you currently have it if the IF condition is not met then there would be no return