Yes, there's enough power as the RX 6400 is only 53w.  As long as it's not the XT version, which is 107w.  Dual fan double-height version is likely a no go.
I have a OptiPlex 5060 and it has a 260w Power supply would it be enough for rx6400 and i5 8500
Would a 260w power supply be enough to run a RX 6400 4GB and i5 8500 ?