So I removed a 2015 Macbook Air's HD from the computer and then using an OWC tool, I plugged it into my Macbook Pro and then erased it completely using Disk Utility. When I plugged it back into the Macbook Air, it goes dark and shows a blinking folder with a question mark. How can I fix this? I can't find any info online with this... I have Mac OS Mojave
Blinking folder with a question mark means it can't find the startup folder, because there's nothing on the disk.
Disk should be formatted as APFS, with a scheme of GUID partition map. Reinstall it in the MacBook and boot while holding down option, command, and R at the same time. This will boot into internet recovery using a downloaded recovery image of the latest supported version of macOS your Mac can handle (10.14.3).
When it finally boots into the recovery menu, you can select the option to install macOS and follow the prompts to install macOS.