That error is/was a random bug I first came across in FM22
to fix it I had to remove the fixture rules for the competition [copy the rules first] that is showing the error - see if the file tests now
if it does then you can add several sets of extra fixture rules, leave them blank unused
pick a higher number than the previous one that the comp used and paste the rules back in but do not use the same number as before
eg if it was fixture rule 6 then use one of the higher blank ones you created, say fixture rule 10 this fixed the issue for me