Gantt Suite for Jira v 2.1.x: Video: How to resolve the issue with the error "Changes cannot be saved"
The two most common cases for the error to appear are shown in the video below. The ways to resolve the issue are described in detail.
Note: In addition to checking for the presence of the start/end fields used in a chart configuration, you might want to verify that the field Time tracking is added as well should you use the time tracking in your Jira.