Since the 7.1 update it will not let you print if you have an empty slot in your CFS. Even if your not going to be using that color. That needs to be fixed. If I’m not using that slot it shouldn’t matter if it’s empty. I had to find an old roll of filament to put in place just to print with a different color.
Hello @user3098917494 
Welcome to the Creality Forum…! 
I am not having this problem using the latest Creality Print and the CFS with a K2 Pro. I have 3 colors loaded now and it will print just fine. Press the Auto Mapping icon in the top right to refresh what colors are in the CFS. Make sure the colors are correct and in the right place.
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You must to assign spool slot to every color of your print to be able to print, even if you print will not use it.
By example, you slice a piece with 4 colors on your config, but your print have only one color, never less you must to assign a spool to the others colors.
In this example I got a blue model and my slicer’s filament are blue green, pink and yellow. but my fcs have Black, withe, grey and brown filaments
So when printing, you must to assign a color to each filament even if will be no used:
in this case the model will be printed using brown.
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