How to return alphanumeric accounting flexfield in sla custom source setup

Hi

We have coded some custom sources for our accounting requirement. Now we need to return whole accounting string through our SLA custom sources while doing the setup in oracle I am not able to simultaneously select return type as alphanumeric and check on accounting flexfield.

Receiving this warning 'Field is protected against update'

But when I am doing setup as return type as number and check on accounting flexfield, it is allowing me to do the configuration.

Can you please tell me how to return alphanumeric accounting string or how to enable the checkbox accounting flexfield in custom source setup window.