The reason we can't do fusion is that it requires using extremely high temperatures as well as extremely high pressures to be able to overcome the electrostatic repulsion that occurs between positively charged nuclei.
Also maintaining of the plasma in a confined state for a sufficiently long duration of time to produce net energy gain is also a major hurdle in doing fusion.
Fusion is basically the process of combing of light atomic nuclei to create heavier nuclei.
A fusion reactor is also hotter than the Sun.
Fusion reactors are designed to be able to achieve and reach temperatures that exceed 100 million degrees Celsius or 180000032 F.
And the Suns Core is 15 million degrees Celsius or 27000032 F.
Fusion reactors on Earth have to have higher temperatures to compensate for the lower pressure and density when compared to the core of the Sun.
The Sun's core reaches around 15 million degrees Celsius which is the temperature that is required for the proton proton fusion process to occur.
In order to achieve the fusion in laboratory settings, scientists have to heat the fuel which is usually a plasma of deuterium and tritium to much higher temperatures which is often over 100 million degrees Celsius.
The higher temperatures are required because the fusion reactors on Earth have lower pressure density when compared to the core of the Sun.