I need fix my return. I need to add a specific amount to line 23 under tax and credits. Where do I complete that at ?

What is the entire exact reject message that you got, word for word, including the code?

You didn't say what form line 23 is on that you have to add to. I assume you are talking about line 23 on Form 1040 (or 1040-SR). You can't just add an amount to line 23. That line is a total of other taxes (besides regular income tax) from Part II of Schedule 2. The specific additional tax has to be added on Schedule 2. Then it will flow to Form 1040 line 23. Exactly how you do that depends on what specific additional tax you have to add. There are about 30 different taxes that can be entered in Part II of Schedule 2. The reject message should identify what specific tax has to be added.