HAT5643   {0} for build {1} cannot be applied to build {2}

The fix you are trying to apply cannot be applied to the installed product. The most common reason for this is you are trying to apply an APAR that is not required for your product build level. For example, you applied and committed CAPAR-1 followed by CAPAR-2 and then tried to apply an APAR that was written for CAPAR-1.