You're using the SharePoint 2010 experience version for your SharePoint Online site.
When you click Edit Web Part for a Chart Web Part that's located on a SharePoint Online site, you receive the following error message:
File Not Found.
To work around this issue, use the Chart Customization Wizard to change the Web Part. To do this, follow these steps:
Browse to the SharePoint Online page where the Chart Web Part exists.
Click the drop-down arrow in the upper-right corner of the Web Part to access the Web Part menu, and then click Data & Appearance.
On the Data Connection & Chart Appearance Wizard page, click Customize Your Chart.
Follow the Chart Customization Wizard to make any needed changes to the Web Part, and then click Finish.
Note If you want to make changes to the Web Part such as Chrome type, Chrome State, or other properties that aren't included in the Data Connection & Chart Appearance Wizard, use Microsoft SharePoint Designer 2013. To do this, follow these steps:
Open the SharePoint Online site in SharePoint Designer 2013.
Click the page where the Web Part exists within the Site Pages section of the Site Objects menu, and then select Edit File.
In the Editor, select the code for the Web Part, and then click WEB PART TOOLS on the ribbon.
Make any needed changes, and then save your changes.
This issue occurs after you receive the latest SharePoint Online service update.
For more information about how to change the appearance of the Chart Web Part by using the Chart Customization Wizard, go to the following Microsoft website: