Please verify the amount you have entered.
That makes no sense in relation to the shipment option. Try different L W H, maybe jiggle with the weight.
For eg, min width on a parcel is 3.5". 9cm is bigger than 3.5", but gets rejected. I have to use 10cm.
For Expedited I have to put in larger dimensions than my parcel actually is, to get the label to generate. I can then trim off the excess paper and the label fits just fine.
I was taught by CP employees, that if the system won't accept the numbers, do not change the parcel, just change the numbers. PayPal shipping is the same, change the numbers.