Correct spelling, explanation: the word column comes from the Latin word columna. In the original word, there is no o before u. Coloumn is therefore incorrect. This mistake may come from the fact that there are differences in spelling between British English and American English. One of such differences is o vs ou spellings: colour (Br En) vs color (Am En), favour vs favor, etc. Column is not one of such words, therefore coloumn is not correct spelling.

Definition of column:
noun, a pillar, often decorative, that supports a fragment of a building; a vertical part of text, for example in a newspaper; a group of people moving in a line, usually in military
The Parthenon in Athens is famous for its impressive columns, which are made of white marble.
The soldiers marched in columns toward the enemy’s position.

Incorrect spelling, explanation: this mistake most probably comes from the confusion connected with differences between American English and British English. One of the rules says that ou spellings in British are only o in American, for example, labour vs labor. Column is not such a case, however, this is why we absolutely always spell it with only u, not ou. Therefore coloumn is a mistake in spelling, and the correct form is column.

Incorrect spelling

Incorrect spelling

Incorrect spelling