To fix the "Mixed Content Error" please follow the below steps:
1: Login to your Wordpress Dashboard, then go Add new Plugin option.
2: Search the plugin " SSl Insecure Content Fixer" and Install/Activate that plugin and open the settings of plugin.
3: Under the Settings of this Plugin and do the settings as choose Capture All option and recommend settings appeared in Red.
4: After doing all the above steps click on Save changes and then your all Mixed Content Error will be Fixed.