Type sticky in Cortana Search > Right click on Sticky notes > Pin to Taskbar
If above doesn't work, Try Repair / Reset of Sticky Apps
Windows Key+i > Apps > Click Sticky Notes > Advanced Options > Repair (If repair doesn't work, Reset)
If still not solved - Uninstall and Reinstall Sticky Notes app (Data will not be impacted)
A. Uninstall - Right click on Windows Start > Click Windows Powershell (Admin) > Copy the below and paste into Powershell and Enter
Get-AppxPackage MMicrosoft.MicrosoftStickyNotes | Remove-AppxPackage
B. Reinstall
Right click on Windows Start > Click Windows Powershell (Admin) > Copy the below command and right click in Powershell to paste > Enter
Get-AppxPackage -allusers Microsoft.MicrosoftStickyNotes | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register “$($_.InstallLocation)\AppXManifest.xml”}
Let me know if you still face the problem. I will provide further steps.