Your purchase code appears to be invalid. Please take special care to enter it correctly without accidentally including spaces or other additional characters. Please note, you can’t activate LayerSlider with your theme’s purchase code. This also can be a temporary issue in our services. In that case please try it again in a few hours.
Is this for a plugin which was bundled within a theme?
If so then you don’t get a license specifically for the plugin. It will still work as required without issue but updates will be handled via the theme author.
More info https://help.market.envato.com/hc/en-us/articles/213762463-Bundled-Plugins