Apple - How do I launch Terminal while in macOS Sierra Recovery Mode?

This is a bug in the recovery Mode, also the same issue if you create a USB Device for install. Try to load Recovery Mode again and choose one the the four available options that should get the menu back.

I've tried boot from USB Device three times until the menu was visible.

After battling this problem for hours I just found a great, reliable solution!

If you reboot holding Command-R-S then you can get into an undocumented combined recovery mode and single user mode, this will go directly to the command prompt without the graphical interface but you will be in recovery mode and can enter the csrutil disable command successfully.

I did this by triggering the reboot from the device selector dialogue in the regular recovery mode on Sierra on a Macbook 2010 but it probably works via other methods.

So happy to have found a reliable workaround for this at last, hope it will benefit others.

You need to click the Utilities menu, as shown in the image below, then click Terminal.

If you do not have the Menu bar showing then you can either try rebooting to see if that makes any difference or just click, e.g. Disk Utility and then close it to see if it will bring the macOS Utilities Menu bar up. _{(As noted by dante12)}

Otherwise try holding down Option-Command-R to start up from OS X Internet Recovery to access Terminal from there.