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You must select mandatory fields, when configuring the transition. For example: 
For information on how to configure a validator in JIRA, see the JIRA documentation.
Include Page Custom Error Message Custom Error Message
Example
A workflow is configured so that the 'Resolve' transition has the 'Fields Required' validator where 'Fix Version' is mandatory. If a user attempts to resolve an issue on this workflow, where the 'Fix Version' field its value not set, the validation will fail and the transition will not be completed.
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