If the child has not stayed overnight with you in 2020 then you are not the custodial parent. The custodial parent is the parent with whom the child has spent at least 183 nights.
Did your ex sign a Form 8332? Why are you claiming the child?
If there is a signed 8332 then the custodial parent retains the right to file as Head of Household, get earned income credit and the childcare credit. The non-custodial parent (you) gets the child tax credit for children under the age of 17.
As far as the IRS is concerned, the custodial parent is the one with whom the child spent the most nights during the tax year--at least 183 nights.