Why didn't the Senate impeach President Andrew Johnson and not the Congress?

The Constitution gives separate powers to the House of Representatives and the Senate. Only the House of Representatives can impeach (bring charges against) a government official. Only the Senate can hold a trial to determine whether the impeached official is guilty. The Constitution requires impeachment activities to occur in that order. This is similar to the way the legal system works: a defendant must be charged first and tried second.

The Senate didn't remove Johnson from office after the House impeached him because they couldn't get two-thirds of the Senate to agree to find him guilty. Johnson was acquitted by a single vote.

The word Congress refers to the Senate and the House of Representatives as a whole, and doesn't apply here. Each part of Congress has its own job.