I have OctoPrint installed on Windows and when I try to connect to my printer I get "Access Denied"

Some other program already has the serial port of your printer opened and Windows prevents you from opening it a second time.

If you have Cura or Simplify3D installed and running when this happens, close them. Both are known to automatically connect to plugged in serial devices and cause these kinds of issues.

In the case of Cura, if you do not also want to be able to print from it and just use it for slicing or remote printing via the excellent OctoPrint Connection plugin, you can disable the bundled "USB Printing" plugin by unchecking it under Marketplace > Installed: