External drive with OS X 10.13.2 will not boot from USB 3.0 enclosure
Assuming I read your question correctly, on the face of it it seems the issue is with the enclosure since neither the HDD nor the SSD will boot when inside it.

However, two things you could try (because they're simple and quick to do) are to reset both the NVRAM and SMC, in that order. Before following the steps below, ensure you have no external hardware connected (so, unplug your external enclosure) and make sure you use the built-in keyboard.

Reset the NVRAM

Older Macs had what's called Parameter RAM (PRAM), newer Macs use Non-Volatile Random-Access Memory (NVRAM). Here’s how to reset the NVRAM on your particular MBP:


Shut down your MBP. That's a full shut down, not just logging out.
Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down until your Mac reboots again and you here the startup chime.
Let go of the keys and let your Mac reboot normally.


Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Reset the SMC

To reset the SMC on your particular MBP, follow these steps:


Shut down your computer
Keep the power cable plugged in.
Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
Let go
Turn your computer back on with the power button.


After resetting both the NVRAM and SMC, test your Mac to see whether you can boot from the external enclosure.