Manage Supplier Bank Account Descriptive Flexfields - Managing the Values in a Value Set

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I am trying to test a new supplier bank account DFF and I want to have it pull from a list of values of 2 items. However, when I try to edit the value set it is pulling from, the "Manage Values" button is grayed out. I've tried to copy the setup of existing DFFs that have a value set I can update, but the button is still grayed out for me. Is there something I am missing in the setup here that is preventing me from defining a list?