"unable to reply" is correct and usable in written English.
You can use it when someone is not in the position to answer or respond to a question or request. Example sentence: "I asked for directions, but the passerby was unable to reply."
"I was unable to reply to their love.
"Generally they put the victim to sleep with a choke hold – locking the windpipe like this," he says, rendering me unable to reply.
I was asked, as usual, what the British government has said to support the students and, as usual, I was unable to reply.
If one-twentieth of Americans, who are 20 points Democratic-leaning, are unable to reply to surveys, Mr. Obama's standing in the polls would be negatively impacted by a net of one percentage point as a result.
National esteem was low for years, because much of the world assumed that the Soviets could launch a nuclear weapon at the United States but that the United States was unable to reply.
Apparently, with Microsoft's latest upgrade of the Windows Live Mail service, things got really broken, causing users to be unable to write, reply to or forward e-mail messages.
Please note that he is unable to answer questions of a legal nature or to reply personally.
Please note that Jeremy is unable to answer questions of a legal or contractual nature or to reply personally.
Please note that he is unable to answer questions of a legal nature or reply personally.
Board members were unable by law to reply to the 20 or more people who made comments because the topic was not on their agenda, but later expressed their appreciation for teachers by officially declaring May 9 through 13 "Week of the Teacher".
