TROUBLE WHILE CHANGING CONCRETE PILE LINK´S DIAMETER - STAAD RCDC

Hello,
I am having trouble while trying to change the diameter of the concrete pile links that appear by default after running the RCDC program.
As seen in the image below, for the given stresses, the program suggests that I use # 19 with a spacing of 100 cm along the entire length of the pile.
However, this is excessive for the low shear stresses in the pile, thats why i´m willing to change the link´s diameter from # 19 to # 13, but here´s where
the program wont allow me to do so and shows this poster:



Thanks in advance!

Ignacio