If you still have trouble installing the demo with the one-click importer you can try to do it manually. Please follow these simple steps.
1. Log-in to your WordPress backend and navigate to Appearance → Import Demo Data.
2. Click on Switch to Manual Import link at the bottom of the page
3. Upload content.xml from 2. Sample Data folder in Import Content
4. Upload widgets.json from 2. Sample Data folder in Import Widgets
5. Upload customizer.dat from 2. Sample Data folder in Import Customizer
6. Once all files are uploaded click on Continue & Import and wait until the demo data is imported.
