Article II section 4 of the US constitution says:
The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.
Articel one section 3 says (in pertinant part):
Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.
Both provisions seem to contemplate that an impeachment and Senate trial would always be of a sitting official, and since it says "removal from office and disqualification" there is an argument that when removal from office is not possible, impeachment and trial cannot lawfully occur. Ther is no clear precedent, and such precedents as exist are over 150 years old.
The Senate could vote to dismiss the articles of Impeachment on this ground, or individual Senators could cite this as a reason not to vote to convict. No one yet knows with assurance what the Senate will do. As the Senate has "sole power" to try impeachments, it is probable that no court would consider the question.