Select the INCORRECTLY spelt word.

The correct answer is Mountan.

Explanation:
Let us check the spelling of each option to identify the incorrectly spelt word:

1. Mountan: This word is incorrectly spelt. The correct spelling is Mountain, which means a large natural elevation of the earth's surface.
2. Weekend: This word is correctly spelt. It refers to the end of the week, typically Saturday and Sunday.
3. Dessert: This word is correctly spelt. It refers to a sweet dish served at the end of a meal.
4. Journey: This word is correctly spelt. It refers to the act of traveling from one place to another.

Therefore, the word with the incorrect spelling is Mountan.