Hey johnburton,

Your activation was most likely deactivated remotely some time ago and the plugin detected this, therefore the error. Please read the following part of the documentation for possible reasons for the deactivation: https://support.kreaturamedia.com/docs/layersliderwp/documentation.html#canceled-activation

In any case, your purchase code is still valid and working so you can activate the plugin with it.   
We have recently launched a new site that you can use to check and reset the activations of your purchase code, please refer to the following url: https://kreaturamedia.com/reset-activations/   
If you reset your purchase code in this site and re-activate the plugin on the admin area as well, there should be no error anymore.