Manage Value set - not able to enter any values

I have created descriptive flex field for create payment UI.

in that Descriptive flex field, I cannot able enter any values in the value set -

->Manage Value Set

Query the Values set name.

Edit the value set that I created. Edit value set opened but manage value button is grayed out.

Please help me, how I can enter values of the Descriptive values set.

Please find in the attachment.

Thanks

Venkatesan