When .NET framework 3.5 is not enabled on client pc, SPU installer try to auto enable it using command to the dism.exe
It may take around 8 - 10 min to enable it, but it is observed on some pc's it takes longer than expected time, may be its pc specific issue.
In this case it is better to restart the pc and then try to install SPU or just turn on this from the Windows feature and then install SPU.
If .NET Framework is not enabled on the client machine, you can enable it by: