This new airport has been in service for six years ago

去掉ago。

This new airport has been in service for six years./This new airport has been in service since six years ago.

两种改法Both OK.

已更新！！！

ago,应该去掉 ago：表示过去时
而你句子中用的has been，又表示现在进行时
一个句子中，前后时态要吻合，因此此句应该改为：
This new airport has been in service for six years.

ago只能表示一个时间点，不能与完成时连用。

This new airport has been in service for six years .

去掉ago
或者把for改成since

不要ago
this new airport has been in service for six years

Ago pass....