5CEBA7F27C8N FPGA FF placement setting

Flipflops and all logic are placed by the Fitter when you compile the design. If you are referring to the registers directly in the I/O elements, referred to as the fast I/O elements, you can enable assignments in the Assignment Editor to make use of them. Other than that, most folks don't manually place individual registers except in extreme cases (like just missing meeting timing).