CC2538-CC2592EMK: How to make association permit true for permanently in ZR ?

In Zigbee 3.0, it's not allow to do permanently permit join.

If I do it by running a loop or creating an OSAL event, is there any issue??

You can do it but it's not recommended.

why it is not recommended??

According to Zigbee 3.0 spec, it's not allowed. And if you enable permit join permanently, any device might join accidentally and cause some problem, like association table is filled by unwanted devices.