You may see Revolution Slider error at the front-end if you have not activated the Revolution slider plugin yet. If the plugin is activated, you may see the slider is not there yet. That means, you need to make a slider using the revolution slider plugin OR import slider zip files (you will find the slider zip files in the ‘slider’ folder of your purchased files) using the import option of the revolution slider plugin and then add that slider into the page using Drag and Drop functionality of Visual composer.
How to Import The Sliders?
Find Slider Revolution menu in the wp admin dashboard. Click on that and you will have options to create a new slider. After creating a slider you will need to create and style the slides. After saving slides you can use the slider anywhere of your website. To use the slider with visual composer page builder you need to just insert the Revolutions Slider using drag and drop feature and select the slider from the given drop-down. For more information about styling a slider you can visit here: http://revolution.themepunch.com/
You can also import slider files (Slider files are included into the ‘sliders’ folder of your purchased theme files.) and use the ready-made slider as you can see in the live demo of the theme. To import slider you need to click on the Import Slider box as you see in the below screenshot and then upload the slider zip file (Slider zip files are included into the ‘sliders’ folder of your purchased theme files.).