"due to gap of HDHP in 2024, the last month rule of 2023 failed, I should return the excess contribution by April 15"
To elaborate on what BillM223 said, failure to complete the testing period does not result in excess contribution for 2023, so there is no excess contribution for 2023 that can be corrected. The fact that the testing period was not completed simply means that you own an extra tax with your 2024 tax return on the portion of the 2023 contribution that is beyond what you could have contributed were it not for the last-month rule.
