Hi @Katarzyna_Sawicka and welcome to the Quick Community!
Based on your screenshot and that the code runs with no errors, my guess is that the code might be missing something that the Gantt setup expects you to include. If possible, can you please provide a screenshot of your code as well as a screenshot of your field wells. Additionally, if you’re comfortable, feel free to post your analyses to the Amazon Arena Sandbox and I can directly interact with your visual to see what the issue is. Here is the link to the Arena if you want to go that route: Amazon Arena Sandbox: An embedded instance of the Quick Sight capability, within the Amazon Quick Community - What’s New - Amazon Quick Community
Lastly, if you have not checked out this resource, I would highly recommend you do so. This resource you can use for reference for most of the popular Highchart visuals, but more importantly, it includes a Gantt chart example with its complete Highchart code (DemoCentral). Just make sure you go to the ‘Samples’ tab at the top to see all the examples. Hope this helps!