Can not save opportunity in

Can not save opportunity when i create opportuinty directly from lead

it shows message

Source cannot be “Website”. It should be one of “”, “Existing Customer”, “Reference”, “Advertisement”, “Cold Calling”, “Exhibition”, “Supplier Reference”, “Mass Mailing”, “Customer’s Vendor”, “Campaign”, “Walk In”

Looks like system treats default empty value of “Source” drop-down list as “Website”.
You should explicitly select the source of opportunity (refer to screenshot).