External USB-C SSD not recognized on MacBook Pro
Since you're using a Beta version of Catalina it is always possible that this issue could be resolved by the final release version of Catalina.

However, assuming this is purely related to a hardware limitation with your current setup, the only way to try and resolve it is to get enough power to the external SSD. As such, you have the following options you could try:


If the external USB-C drive has the option for its own power source connected to it, then doing that will solve the problem
You could opt for an adapter that connects from two USB-A ports to one USB-C port, thereby combining the power from two ports to provide enough for the external drive
You could use a powered external USB hub or dock (i.e. connect the USB Hub to the MBP and then connect the USB-C drive to the hub)
Double-check that the adapter you're using is actually capable of passing enough power through


NOTES:


While there are many cables/adapters that can utilise the power from two USB-A ports to a single USB-A port, I'm not sure if one exists that goes to a single USB-C port - so you may need a second adapter to get this to work.
Only you can decide if the expense of using a hub or dock is warranted in this use case.