The Apple adapter to convert Thunderbolt-2 <-> Thunderbolt-3 devices supports ONLY genuine ThunderBolt devices on each side, which your drive is NOT. it costs about US$50, and requries an added thunderbolt-3 cable, bringing the price to over US$100, and it will not work for your drive.
It's 2026, and in hindsight, we now know that Thunderbolt-2 today is good for DISPLAYS, and not much more. The market never provided wickedly fast Thunderbolt-2 devices other than displays.
To connect with all but the absolutely most demanding drives, use the USB ports implemented on your 2015 MacBook Pro, which provides USB-3 up to 5 Gbits/sec using a USB-A connector. Add adapters as needed.
If you have run out of USB ports, you need to add a USB Hub.
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Your dilemma pales by comparison to owners of the 2103 Mac Pro, which has SIX Thunderbolt-2 ports:
