i am new one for coding in ARM

While you report being "new" - you've discovered a special requirement of this vendor's newer devices.  Many other, "new" users have made that same discovery - in their case most often because they've failed to achieve the desired behavior from that specific MCU pin.

That pin is PF0 - and it - and one other - default into a special format - which (it would appear, to the "new" hundreds who've been so victimized) to be inadequately highlighted and/or safeguarded by all existing vendor publications.

Code block you show attempts to unlock that pin - but does not carry on and, "re-purpose" the pin.  (that - being the, "raison d'etre" for the unlocking...)

This vendor - I believe - does quite a good job in providing illustrative & generally useful code examples - often with detailed commenting. 

Suspect that a more systematic approach would speed & enhance your, "ARM coding & understanding."  Issue you raise impacts 2 out of 64 (or more) MCU pins - I'd suggest your avoiding that pin for now - and focus instead on first reading & then experimenting with the various MCU peripherals.  The peripheral driver user's guide, the MCU manual, and the many sound, code examples provided will certainly keep you busy - and aid your development.

While at sea - at night - many "interesting" points of light may register.  Yet - efficient sailors know to focus upon the key ones - no matter how enticing - the "unusual/unexpected" ones appear...