I trying to setup a validation rule so it forces the users to only select from the picklist and not type in whatever they want. Here is what I have so far but it gives me an error.

CASE(Subject,
"CPAP Trial", "CPAP Trial",
"Pressure Change", "Pressure Change",
"Mask adjustment", "Mask Adjustment",
"Equipment Adjustment", "Equipment Adjustment",
"CPAP Purchase", "CPAP Purchase",
"Supplies", "Supplies"
"APAP Trial", "APAP Trial",
"APAP-CPAP", "APAP-CPAP",
"Download", "Download",
"Other", "Other"
"Call", "Call"
"Email", "Email"
"Meeting", "Meeting"
"Send Letter/Quote", Send Letter/Quote"
"None")

Error Message:
"Please select items from picklist"