Question

why is fusion nuclear energy currently not widely used as a source of electricity?
fusion reactions have not yet been achieved.
fusion reactions cannot be controlled.
fusion reactions require very high temperatures and pressure.
fusion reaction energy cannot be converted into electricity.

Explanation:

Fusion reactions need extremely high temperatures (millions of degrees Celsius) and pressures to overcome the electrostatic repulsion between positively - charged atomic nuclei. Currently, achieving and maintaining these conditions in a controlled and sustainable way for large - scale electricity production is very challenging.
Fusion reactions have been achieved in experimental settings (e.g., in tokamaks like JET). Also, if fusion reactions could be controlled, the energy could potentially be converted into electricity (similar to how fission energy is converted). And while controlling fusion reactions is difficult, the main limiting factor at present is the requirement of high temperatures and pressures.

Answer:

Fusion reactions require very high temperatures and pressure.