To resolve this you must have permissions to that Next-Gen project.
Go to Project list,
2. Find your Next-Gen project, click Project Settings → Issue Types.
3. Select any Issue Type and create the fields with the below specified format.
RQ Start DateTime
Timestamp, an optional field, create it only when you don't want to use the Jira's default Start date field.
RQ End DateTime
Timestamp, an optional field, create it only when you don't want to use the Jira's default duedate field.
4. Once created, your fields will be available in the Previously-Created Fields section as below. Also, added to one of the selected issue type.
Make sure to include these fields in all Issue Types like Epic, Bug, Task, Subtask, etc,. Check the warning in the Gantt once again, whether you missed to configure any issue types.