Transfer To The PLC Is Not Allowed (icon gray)

The "Transfer to PLC" icon will be disabled (grayed out) and Treeview icon include a small arrow if you have used the "Partial Transfer" option: 

Fig. "Transfer To PLC" icon disabled

Fig. Small arrow icon

This is as described in the manual:

CX-Programmer Operational Manual W446-E1-14 (page 24):
Programs can be downloaded to CS/CJ-series CPU Unit Ver. 2.0 or later, and downloading by task (program) instead of the whole user program is possible (CX-Programmer Ver. 4.0 or later). Select Partial Transfer – Task Transfer to PLC from the PLC Menu, and download a specified single task or multiple tasks. This function enhances the efficiency of program development by multiple personnel. Particularly, by downloading to the PLC by task (program), only the parts to be changed are reflected in the PLC, resulting in fewer working errors.

A partially reassembled object cannot be re-transferred. To resolve this problem, first upload the whole program from the PLC and save the project. Now you can download again.