Append to XML remains greyed out

Hi folks,

I am using MII 15.0 SP2 Patch 8. I created a new transaction having a xML transcation variable called "test" defined as you cans ee in below screenshot. It is a plain XML having a root node called options.

Thereafter I created a new transaction adn placed an assignment block there. I treid to use append XML link type to extend my transaction variable. Unfortunately, when trying to append some new xml to test{/OPTIONS}, append XML functions remain greyed out...see screenshot below

Any idea why that happens? Thanks for your help!