Near the end of July 2017, Google Chrome created a plan to first reduce and then remove trust (by showing security warnings in the Chrome browser) of all Symantec, Thawte, GeoTrust, and RapidSSL-issued SSL/TLS certificates. Google broke this timeline up into 3 important dates. December 1, 2017, March 15, 2018, and September 13, 2018. The first date, December 1, 2017, required no action from you. However, for the 2018 dates, you must replace affected certificates to avoid Google Chrome browser security warnings.
For more information, click here: Replace Your Symantec SSL/TLS Certificates