Are there some elements on the periodic table that hasn't been discovered?

No, we've found every naturally made element. There are currently only 92 naturally made elements on the periodic table. All the elements with more than 92 protons are synthetic. There are new elements that can be added to the periodic table. But these elements are man-made. There are no more elements to be discovered.

We can theoretically create as many elements as we want in laboratories.

All the naturally existing elements on earth have however been discovered.