Control 5V relay board with Raspberry PI GPIO
The IN must be pulled down to ground to turn on. It must be pulled up to VCC to turn off. At 3.3V IN  there is still a difference of 1.7V and that may result in current through the optocoupler so it may not turn off. And when HiGH-Z or ground the gpio would send 5V potential through it.

As long as you are sure VCC and JD-VCC are isolated, then VCC should be 3.3V for the RPI to work with it. But it could also not be enough for the opto coupler and the status led to light up. Alternatively would can use a NPN transistor (2n3904 is fine) from the RPi gpio to control the Opto. This would isolate the IN so you can power the relay board from 5V.