apply invoice to guarantee

we are exploring how to use guarantee:

steps:

1) create a invoice transaction type - INV-TYPE-1

2) create a  guarantee transaction type - GUA-TYPE-1

3) create a guarantee invoice - 100 (GUA-TYPE-1)

4) create a invoice 101 (INV-TYPE-1)

then in the transaction screen, we query out invoice 101, and go to Action->Application

but the 'Application' menu is dim out, what have we did wrong?

we are using R12.