Solution
OPC UA Toolkit 2018 and later
Starting with version 2018, OPC UA Toolkit supports LabVIEW 32-bit and 64-bit. This is also reflected in the readme of this software.
OPC UA Toolkit 2017 and previous
The OPC UA Toolkits versions 2017 and prior do not support LabVIEW 64-bit. In order to work around this limitation, you can try to split your application in two parts, and have them communicate using TCP/IP using the localhost IP address (127.0.0.1) or any other inter-process communication method that works regardless of the bitness. You can read this article for more information about TCP/IP communication using LabVIEW : Basic TCP/IP Communication in LabVIEW