Do I need a crank arm fixing bolt?
If the bolt in the photo is the same as the one that’s missing, it’s to preload the bearings before tightening the pinch bolts on a HT2-style crank.
If that’s the case, I’m not sure what consequences, if any, are likely from riding with just the pinch bolts and no preload bolt. I would think it’s similar to riding without a headset cap in that once the pinch bolts are set, it’s not completely necessary, but I’m not 100% sure it’s safe for the parts on a crank- there might be some repercussions I’m not thinking of.
I wouldn’t deliberately ride a HT2 crank sans preload bolt any longer than it takes to get to the shop for a replacement, but the implications of riding without it are not the same as for other styles of cranks where the fixing bolt torque actually holds or presses the crank onto the spindle.