Change:
If(AccredList.size()>0)
return 'Opportunity Number is ' + AccredList;
To something like:
If(AccredList.size()>0){
return 'Opportunity Number is ' + AccredList;AccredList.size();
}
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