Lock Screen slideshow not working

Ok, Windows 10 1903 will soon be released, you could try upgrading to that release to see if it is fixed.

or

Press Windows key + X
Click Windows PowerShell (Admin)

At the prompt, type the following command or copy and paste it then hit Enter. Exit then restart.

Get-AppXPackage -AllUsers |Where-Object {$_.InstallLocation -like "*SystemApps*"} | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}

1. In Start > Settings > Personalization > Lock screen change the lockscreen to "Picture"
2. Toggle "OFF" the switch for "Get fun facts, tips, tricks, and more on your lock screen" in Start > Settings > System > Notifications & actions
3.
Open Start, type: CMD
Right click CMD
Click Run as administrator

Type in at the prompt OR Copy and Paste these one at a time : (Hit enter after each)

Dism /Online /Cleanup-Image /CheckHealth

Dism /Online /Cleanup-Image /ScanHealth

Dism /Online /Cleanup-Image /RestoreHealth

Also run the System File Checker utility:
Open Start, type: CMD
Right click CMD
Click Run as administrator

At the Command Prompt, type: sfc/scannow

This will check for any integrity violations

Restart your system

Do this again

Press Windows key + X
Click Windows PowerShell (Admin)

At the prompt, type the following command or copy and paste it then hit Enter. Exit then restart.

Get-AppXPackage -AllUsers |Where-Object {$_.InstallLocation -like "*SystemApps*"} | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}

Note that I still got the same error about Cortana app needing to be closed.

Click Start
Scroll down to Cortana
Right click Cortana
Click More > App Settings

Click Terminate

4. Restart Windows 10

5. In Personalization I changed the lockscreen back to "Spotlight"

Check if it now works.