Kathleen Sebelius, the secretary of health and human services, said Wednesday that the Obama administration would not extend the deadline for people to sign up for health insurance or delay the requirement for most Americans to have coverage. And she declined to say whether the administration was still committed to its original goal of enrolling seven million people in private coverage through federal and state exchanges by March 31. Testifying before the House Ways and Means Committee, Ms. Sebelius said categorically that the administration would not delay the "individual mandate," under which most Americans must have insurance or pay a tax penalty. In addition, she said that officials would not extend the six-month open enrollment period, scheduled to end on March 31.