Hello,
But now when i check the MPC5748G RM, i didn't see any chapter describes how to set the Pin to enter the serial boot mode.
No such pin is present anymore.

Does it mean i don't need to set the external pin ?
No external pin is needed / present on this device. See description above.
If i understand correctly, does it mean if i set the the lifecycle to "In field" , i cannot enter serial boot mode any more ?

Yes, you cannot.
So, i cannot use "Rappid bootloader" to download the code to internal SRAM ?
Hmm, Rappid bootloader is (flash resident bootloader) running from internal Flash if I am correct. So you will boot from internal flash and then your bootloader will take care of update via serial interface.
It is much better to use resident bootloader then this serial boot. Also necessary for in field, where you need to do SW updates.
Best regards,
Peter