1. Delete com.apple.Safari.plist file and test.
Empty the Trash.
Quit all applications.
Hold the option key down and click the "Go" menu in the Finder menu bar.
Select "Library" from the drop down.
Library > Preferences > com.apple.Safari.plist
Right click on it and select "Move it to Trash".
Close the Finder window.
Restart and relaunch Safari. Click “Start using Safari” link at the bottom.
If this doesn’t help, “Put Back” the com.apple.Safari.plist
Right click on the Trash icon in the Dock and select “Open”.
Right click on the com.apple.Safari.plist and select “Put Back”.
2. If Step.1 doesn’t help, backup your Mac and reinstall macOS
Reinstall macOS.
Choose the appropriate key combination.
How to reinstall macOS. https://support.apple.com/en-us/HT204904