. . . . . . . . common sense is . . . . . . . . virtue.

'Common sense' is an abstract noun. Use of article is usually omitted before abstract nouns. So, no article precedes this word.
The word, 'virtue' is also an abstract noun. However, there are many 'virtues' in the world. 'Common sense' is one among them. Hence, the indefinite article, 'a' is to be used before 'virtue'.