In order to register to a booster, the sum of your MRF (plan) and the promo you are availing should not exceed your credit limit.
Your plan is 999, and you're registering to super unli 599, 999+599=1598. In order to process this, the credit limit must be higher than 1598.
Request for increase in Credit limit can be done through Hotline, or in a Globe store to have a possible realtime increase in CL
I have the same problem. My plan is 999 with 1793 credit limit.