Explanation
The best way to create a field that shows the amount as described in the question is to use a bucket field off of Amount. A bucket field lets you group report values into categories without creating a formula or a custom field2. You can create a bucket field for any numeric, picklist, or text field in a report.

A validation rule on the Opportunity object is the best way to enforce the requirement that the Reason Lost field is required when the stage is changed to Closed Lost. A validation rule can specify a condition and an error message to display when the condition is not met. For example, the condition could be AND (ISPICKVAL(StageName, "Closed Lost"), ISBLANK(Reason_Lost__c)) and the error message could be
"Please enter a reason for losing the opportunity." Making the field required on the page layout or universally required would not work, because the field should only be required when the stage is changed to Closed Lost, not for other stages.