It looks like you are trying to run an AC valve from DC. It will draw quite a bit of current at 24VDC. Ray has written some on the subject. It may be that the valve loads the power supply enough to reduce the voltage too much for the relay to operate. I suggest you measure the 24V supply with the valve on.
https://rayshobby.net/wordpress/understanding-24vac-sprinkler-valves/