GFI Leviton installation corrected
The most common reason for not being able to reset a GFCI after performing work on the circuits is miswiring the GFCI. Verify both your source or line side circuit neutral and hot conductor are connected to the Line side of the GFCI and the Load side circuit neutral and hot conductor is connected to the correct terminal. The neutral must be separated and fed though GFCI device also. So no pigtails. This is a common mistake and has happened to many professionals myself included. 

I can't definitely say this is the problem but it is a good start. Good luck.