Change system photo library in Photos but no response

I would like to change the system photo library in Photos to a folder in my external drive.

You don't designate a folder as the system library. You need to create a new Photos library, put it on the external HD, open it with Photos, go to the Photos/General preference pane and click on the Use as System Photo Library button. When the button becomes greyed out it is the System Photo Library.

However, the external HD must be formatted properly as Larry as pointed out.

what is the format of the EHD and how is it connected? Unless it is properly formatted adn connected you can not make it the system library - Where is it safe to store a Photos Library? Requirements for the external drive

And although it does not affect this you are three updates behind and missing lots of new features and bug fixes - unless there is a good reason not to it would be a good idea to get up to date with High Sierra

LN