I think it depends upon how you purchased it. If you bought a boxed DVD version, you will have installers for both Mac and Windows. If you purchased a download from Adobe, I think you only get one or the other. If this is the case, you may be able to contact Adobe support and have them change it for you. You can start a chat session with Support by clicking on the blue Contact Us button under the Support menu at Adobe.com. Or you can contact Support by telephone using the numbers listed here: https://helpx.adobe.com/contact/phone.html.
@Greg_S. is spot on regarding the physical boxed product of Adobe Photoshop Elements 2021 being dual-platform, but the most recent versions of Photoshop Elements that you can buy as direct downloads from Adobe are also dual-platform too 🙂
To double-check that, just go to your Adobe ID account page and under Plans > Order history you should see your Adobe Photoshop Elements 2021 order. Click on the order number, then in the Items section click on the Downloads and serial numbers link. This will show your serial number (which is the same for both Mac & Windows) and give you two download links - one for Mac and one for Windows 🙂
Here's mine (for the Photoshop Elements 2021 & Premiere Elements 2021 bundle) :