changing from single to mutiple selection

changing from single to mutiple selection

Hello I have being testing out the Library App here is my question when I attempt to switch the field from single to mutiple selection I get this popup which say's

This field is referenced in the following places. Please remove them and then change the field type.

Script Actions.

1.on success
2.on success
3.on submit
4.on change

what I'am I doing wrong can you please advise on how to go about chaging something like this I'am changing this in the Isuue Retrun section of the Library App Iwould like it be able to select more than one item to return thanks in advance.

Regards,
brabago