Can you take the road test for cdl in box truck under 26000lds?

The only way it would be useful for you in any way is if it had air brakes, and you were going for a Class C CDL and wanted the air brakes restriction lifted. Class C is for vehicles under 26,001 lbs. GVWR which are either transporting hazardous materials (and thus require a hazmat endorsement) or are buses designed to transport more than 15 passengers (this includes the driver, and would also require the passenger endorsement, plus it would require the road test to be taken in an actual bus).

For a Class B CDL, it is of no use to you, whatsoever. For a Class A, if you attach a trailer with a GVW of more than 10,000 lbs. to it, you can get a Class A with that, but you'll have a "no tractor-trailers" restriction.