Before you can upload a Voice Memo to your Rev account, you need to save it to your device. Open the Voice Memos app on your device and tap the file you want to have transcribed. Tap the ellipsis menu (...) to open the file options.
Tap Save to Files
Tap the On My iPhone option to save the file on your device, and then select the folder where you want to save the file. Once you're done, tap on Save.
Now that your file is saved directly to your device, you can upload it to your Rev account by tapping Upload File and then navigating to the folder where you saved the file and tapping on the file.