What conditions are required for nuclear fusion that prevent it from being used to generate electric energy?

Answer: the extremely high energy and temperature needed to start and keep the reaction.

Explanation:

Nuclear fusion is the combination of atomic nulei.

Those reactions are able to release huge amounts of energy.

Sun energy comes from fusion reactions, mainly the combination of two hydrogen atoms to form one helium atom. As said with the release of energy.

Nuclar fusion requires extremely high energies to start and keep a reaction. That energy can only be achived with temperatures greater than 40 million kelvins, which so far has been achived only with atomic bomb explosions.

Also, the materials to handle those temperatures, when you talk about confining the reaction, are not available, so far.