The Add and Edit Opportunity forms have an optional custom validation that can only be enabled by a system administrator.
- Go to Administration >> Applications >> Features > Data Form Search
- Seach for Record Type = Opportunity and Mode = Add.
- Find the Opportunity Add (Custom) form.
- Click Custom Validation in the upper left.
- Check Enable Custom validation and then OK.
- Repeat for the Opportunity Edit (Custom) form.

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