CLKI is the input clock you feed a PLL module with, such you get a CLKOP out of it.
PLL module only multiplies the CLKI by X.
You have to connect an external oscillator (3.3V level) to the CLKI, or use an internal clock source, if any, and connect it to the CLKI.
In the configurator you have to input the actual CLKI frequency, for example 20MHz.