Hey Neha, are you inputting '88' directly as indicated in the screen shot? If so, I don't see where you indicated the number range as an external number range. If you want to control the numbering, you need to toggle the 'Ext' column for 'X' for the interval. Otherwise, just go to the transaction SR10 and select ENTER and a number from the range specified above will be allocated.
Regards,
James