For FCU mode transition, you should set the flash P/E mode entry register (FENTRYR).
When you would lie to set the EFNTRYR, be sure to dummy read it right after writing the FENTRYR setting value.
Then, write ROM/FLD, delete and read.
You perform this dummy read only once after writing FENTRYR.
Accessing to ROM/FLD without FENTRYR dummy read after writing FENTRYR may generate an access violation to ROM/FLD because the FCT mode has not switched yet.
The access violation leads to FCU command lock status which doesn't allow the FCU command to be accepted later on.
To clear the FCU command lock, you need to issue the status register clear command setting the value of the flash access status register (FASTA) to H'10.