Fatal error: Call to undefined function is_woocommerce_active()

Currently we are aware of two possible causes for this error

Outdated plugins

Check that all of your WooCommerce.com plugins are up to date, including WooSlider if you have it installed.

If you have activated your keys, then you will receive automatic update notifications. More on keys and subscriptions can be found here : https://woocommerce.com/my-account/my-subscriptions/

If you have not activated your keys and are using WooCommerce 2.0.5 or higher you can check if there is an upgrade available for any of your active WooThemes plugins by checking the System Status page in your WordPress admin ( WooCommerce->System Status ), if you are on a version of WooCommerce below 2.0.5 then you will need to check manually by looking in your WooCommerce.com dashboard https://woocommerce.com/my-account/downloads/

If you are not on the latest version you can download it from https://woocommerce.com/my-account/downloads/

To update the plugin manually you should unzip the download and load the files via FTP and just overwrite the existing files, there is no need to deactivate the plugin and no need to delete any files. You can’t manually update WooCommerce.com plugins via your WordPress admin unless you have activated your keys, you must do it via FTP.

Third-Party Themes

Some themes have been known to create this error. If all of your WooCommerce.com plugins are up to date, you should swap your theme to Twenty Sixteen and try to activate the plugin again. Once it activates you will be able to swap the theme back to your chosen theme

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