The red light on the Linksys extender can be due to a number of reasons or technical glitches. The most probable things can be an uninterrupted power supply, connectivity issues, and the firmware's outdated version. So, check out these points earlier to move to the further troubleshooting process. Cross-check whether the power socket is working and if that is able to provide uninterrupted supply or not. This can happen when the power socket is broken or has an internal technical bug. Later, try to verify the connection strength between your extender, existing router, and networked device. 

It is critically necessary to have a strong connection between them during the Linksys wifi extender setup or after it. So, there will be strong signals transferred from the router to the extender and your device. If still, the red light keeps occurring on the extender, check for its firmware updates. Because the outdated firmware can make the network sluggish and cause errors in it which shows the red light. Go to the web management page and carry out the updating process if there is any new version available. Alternatively, you can activate the firmware Auto-Update option in the Linksys WiFi app.