Why is there no article in front of the sentence?

thanks in advance.
There is no indefinite article with the word "coffee" because "coffee" is an uncountable noun. However, you could say this:

- Afternoon is the time I need a cup of coffee.

The word "afternoon" does not have an article because that word refers generally to a time of day.
In theory, you could also say "the afternoon", but it isn't necessary in your sentence.
Thanks for the reply.

But what I wanted to ask is not coffee.

Your answer gives me a new question. I thought 'in front of the sentence' meant

'in the begining of the sentencce.' that is, before 'Afternoon' unlike 'in the afternoon'

Am I wrong?