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A screenshot would really help, since I'm not entirely sure what you mean. Are you stuck on the window where the plots are listed? Are there just no plots to select? (That can sometimes be the case).
 
I had this once as well.

I had a plot active to remove a duke that didn't support my heir choice. I've already had some of my vassals backing it, when another duke rebelled and this duke joined the rebellion. They lost and as traitors both lost their titles. Now, my plot was stuck as it was ongoing and I couldn't end it, on the other hand, that duke already didn't have the title as it was revoked. So, I had this plot until the king died.