I hope you are programming CY8C4248lqi_bl583 with an external programmer. SET_IMO_48MHZ API is not required for CY8C4248 BLE devices. This is because target does not require to set the IMO frequency to 48MHz before flash write operations.
I know that this device CY8C4248lqi_bl583 erased data is 0xFF.
And then, after I erased, I check the data as 0x00.
Which one is correct 0x00 or 0xFF?
The default value in PSoC flash is 0x00. That is why you are seeing that value.
Additionally I would recommend you to create new threads on queries which are independent from the initial query.