Ellen White never described Christ’s atonement as incomplete. On the contrary, here are two statements representative of her teaching: SDASBC 8.1
“We are to rejoice that the atonement is complete; and believing in Christ as our complete Saviour, we may know that the Father loves us, even as He loves His Son.”—The Review and Herald, November 11, 1890 (emphasis supplied).
“Do you realize your sinfulness? Do you despise sin? Then remember that the righteousness of Christ is yours if you will grasp it. Can you not see what a strong foundation is placed beneath your feet when you accept Christ? God has accepted the offering of His Son as a complete atonement for the sins of the world.”—The Youth’s Instructor, September 20, 1900 (emphasis supplied).