This is almost always a driver issue. Reinstall the Rockchip Driver Assistant and try a different USB port (preferably a USB 2.0 port).
Comprehensive Guide to RKAndroidTool v1.35: Flashing Rockchip Devices RKAndroidTool v1.35
Unlike standard flashing utilities that only push a single, monolithic image file, RKAndroidTool interacts with the Rockchip device at the partition layer. This is almost always a driver issue
This means you can flash individual components of your Android system—such as the kernel, recovery, or system partition—without wiping the entire device. Version 1.35 is particularly favored for its stability on older Windows systems and its compatibility with a wide range of "Golden Era" Rockchip devices. Key Features of Version 1.35 monolithic image file