〜たことがある: talking about experience
Asking about experience differs from asking what happened on a particular day. Use ta-form + ことがある for experience and compare it with an ordinary past statement.
State that you have had an experience
富士山に登ったことがあります。 means 'I have climbed Mount Fuji before.' Add ことがあります to 登った, the ta-form of 登る, to state from the present viewpoint that the experience has occurred.
You can add a count, as in 三回登ったことがあります ('I have climbed it three times'). The pattern does not itself mean exactly once; distinguish having the experience from specifying how often it occurred.
Say that you have not had the experience
まだ行ったことがありません。 means 'I have not been there yet.' Negate the experience expression by changing あります to ありません, leaving 行った unchanged.
行かなかったことがあります instead means that there has been an occasion when the person did not go. Distinguish never having gone from having experienced an occasion of not going.
Compare specific past events and occasional actions
昨日、寿司を食べました。 reports a specific action yesterday. 寿司を食べたことがあります。 reports having tried sushi. Choose according to whether the question concerns prior experience or a particular meal.
With the dictionary form, 寿司を食べることがあります。 can mean that the person sometimes eats sushi. The contrast between 食べた and 食べる distinguishes experience from occasional occurrence, so check the preceding verb.
Check: Which means 'I have never been there': 行ったことがありません or 行かなかったことがあります?
Show the answer and why
行ったことがありません is correct. The other sentence says there was an occasion of not going, which is different from never having been there.