I mistakenly deleted some code from the start up for Safari. When I try to open Safari now, it puts up a screen message Saying "Safari can't find the page". How can I rectify this problem?

You can reinstall OS X (without erasing the drive) or if you have the installer application you can use locate the package that installs Safari and reinstall it alone using Pacifist.

Reinstall OS X Without Erasing the Drive

Make sure you have a current backup before proceeding.

1. Restart the computer and after the chime press and hold down the COMMAND and R keys until the Utility Menu appears.
2. Choose Disk Utility from the Utility Menu and click on the Continue button. Repair the Hard Drive and Permissions as follows.
3. After Disk Utility loads select the indented (usually, Macintosh HD) entry from the the side list. Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button.
4. When the process finishes, quit Disk Utility and return to the Utility Menu.
5. Select Reinstall OS X and click on the Continue button.

You could try this first: Download and Reinstall OS X El Capitan 10.11.4 Combo Update.

Safari - Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues

Safari doesn't load a page or webpage items are missing

Safari/other browsers – Website not loading