At what input frequency of an LCR meter should I measure the inductance of a solenoid valve?
I think that there is something wrong either with the LCR meter or with the way you used the meter.

It is extremely unusual to see significant capacitance on a solenoid valve.  Let's call that indicator #1.

The resistance value should be the same no matter what frequency was used.  This is assuming that the LCR meter measured the resistance at DC.  Let's call that indicator #2.

What DC resistance did you measure with a DMM?  If you haven't done that yet, please do so when you get a chance.

The datasheet says that the solenoid consumes 7.0 VA (AC) or 7.0 Watts (DC).  That value and the DC resistance that you measure will let you figure out what voltage the solenoid needs if you don't already have that figure.

I normally measure inductance using a fast MOSFET and pulse generator.  I simply monitor the current on a scope as I increase the pulse width.  Given the DC voltage that I'm feeding into the coil and the slope of the trace on the scope (showing current), it's trivial to calculate the inductance.