Some months ago I purchased the Gantt program but it appears that I do not have the full version only the trial version, would you help me solve the problem as I am unable to create charts

There is no such thing as “trial version” of GanttProject. Any download package which is available from our site, GitHub pages or from Gumroad is a full package.

What do you mean when you say “I am unable to create charts”? When you start GanttProject, you already have a project with an (empty) chart.