how do I download office 365 to my MacBook if I don't have Mac 10.15

Microsoft only supports Microsoft 365 or Office 2021 for Mac on macOS Catalina or later. You need to upgrade the Mac to Catalina 10.15.7 via How to get old versions of macOS (using strictly Safari browser). Check if your Mac model meets the Catalina hardware requirements. Catalina only supports 64-bit applications, so upgrading to it may leave behind any 32-bit applications for which you are currently dependent. Use the free Go64 application to get a list of your current 32-bit applications.

If you still cannot upgrade to Catalina due to application or hardware requirements, then you will have to forego MS Office for another compatibility choice. I suggest you trial SoftMaker's Office 2021 (clone) and see if it meets your needs. This has a more user-friendly application interface and MS Office document compatibility without the interface and learning curve issues with the free LibreOffice Suite (MS Office clone).

If your mac is running an OS older than the required minimum OS for Office 365 then you will not be able to run Office 365.

Which MacBook do you have, model and year please. Click the top left and select About This Mac.

You may be able to upgrade your macs OS to macOS Catalina or newer.