My apologies, the linear periodic is only supported in one direction. Refer to post:
/forum/forums/topic/linear-periodic-bcs-in-both-x-and-z-directions-causing-meshing-error/
You can try using a frictionless support on the boundaries of the other direction. This sets up essentially the same constraints as a mirror condition in a structural analysis. A mirror condition is not the same as linear periodic but it may suffice depending on the loading.
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