inevitable

  1. Therefore, the involvement of the government is an inevitable and necessary in all circumstances.
  2. Therefore, the involvement of the government is inevitable and necessary in all circumstances.
    Which of the sentence is correct?

I think #1 is wrong - you need a noun to use “an”.
Inevitable and necessary are adjectives - they don’t qualify.

The second is correct.
“Inevitable” is an adjective in these sentences.
To use the article plus inevitable, there needs to be a noun. For example:
…the involvement of the government is an inevitable consequence and necessary in all circumstances.

I would refine this further and write: …the involvement of the government is an inevitable and necessary consequence in all circumstances. Or alternatively:

…the involvement of the government is an inevitable consequence and is necessary in all circumstances

I think the second one is correct, the first one doesn’t sound good.