Unable to mount external drive on MacBook Pro

such a drive needs adequate Power and a good cable connection to your Mac to be shown in Disk Utility.

In Disk Utility use the tiny View menu and choose "show all devices".

Select the topmost (not indented) item of all that refer to that Physical device.

A drive that will not reveal its make&model, and a reasonable non-zero size/capacity, can not be used on this Mac in its current condition.

We only know what you tell us and right now that isn't much.

Is the drive turned on?

What format drive?

What kind of drive?

Do you see an error message?

Can you see it in Disk Utility?

Have you used this drive before?