Is it legal to make a fire weapon with a 3D printer?
Sort of.

Fully plastic guns are banned in the United States by the Undetectable Firearms Act, because they can pass unseen through metal detectors - a huge problem. One way around this is to insert a slip of metal into the gun, thereby making it detectable by metal detectors. 3D-printed guns made of metal - were that possible - would not violate the law.

The UFA was originally enacted in 1988, extended from 2003 to 2013, and then again until 2023. It may seem a bit unnecessary, as all-plastic guns, like the famous Liberator have structural issues that make them extremely difficult to fire when compared to a normal all-metal gun. However, then can be used.

In Europe, gun laws vary widely by country. The European Firearms Directive is the main international agreement on gun laws, but its application varies, and it does not directly address 3D-printed guns.

I'm not aware of a country that has an explicit law banning 3D-printed guns. The only countries that ban them either unintentionally cover them or ban guns altogether.