This one is tricky, but the expert @HelenC12 is mostly right.
1. For most tax items, you claim the expense when you actually pay it.
2. However, the energy credits are slightly different. In this case, the credit is for "10 percent of the amount paid or incurred by the taxpayer for qualified energy efficiency improvements installed during such taxable year" (emphasis added). So you could claim the credit for items that were installed in 2020 (the cost was incurred) even if you didn't actually pay the cost until 2021.
3. But in your case, the doors were not actually installed until 2021. So you can include the entire cost of the windows that were installed in 2020 (even though they weren't fully paid for until 2021). But you can't include anything for the doors, because they weren't installed until 2021.
4. As far as I know, this credit expired 12/31/2020. Congress has made a (bad) habit of extending the credit at the last minute in 1 or 2-year increments. So pay attention at tax time for 2021. The doors may be eligible if Congress passes another extension in time.