Your structure has the insatiability condition arsing out of two reasons.
(1) The support is pinned and the structure is laterally unstable as as no lateral frame,bracing or struts exists.
You need to laterally confine the support along the lateral direction by either fixed support or fixed but support.
(2) The plates are simply sitting on the member tip by a single node and this arrangement results in instability due to spurious degree of freedom. You may add a small dummy beam inside the plate connecting the column tip.
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