I found this in the build attempt:
Unable to cache built core, please tell STMicroelectronics:stm32@2.0.0 maintainers to follow http://goo.gl/QdCUjo
Linking everything together...
and this at the end:
"C:\\Users\\William\\AppData\\Local\\Arduino15\\packages\\STMicroelectronics\\tools\\xpack-arm-none-eabi-gcc\\9.3.1-1.3/bin/arm-none-eabi-objcopy" -O binary "C:\\Users\\William\\AppData\\Local\\Temp\\arduino_build_323229/Blinktesrt.ino.elf" "C:\\Users\\William\\AppData\\Local\\Temp\\arduino_build_323229/Blinktesrt.ino.bin"
"C:\\Users\\William\\AppData\\Local\\Arduino15\\packages\\STMicroelectronics\\tools\\xpack-arm-none-eabi-gcc\\9.3.1-1.3/bin/arm-none-eabi-objcopy" -O ihex "C:\\Users\\William\\AppData\\Local\\Temp\\arduino_build_323229/Blinktesrt.ino.elf" "C:\\Users\\William\\AppData\\Local\\Temp\\arduino_build_323229/Blinktesrt.ino.hex"
Using library SrcWrapper at version 1.0.1 in folder: C:\Users\William\AppData\Local\Arduino15\packages\STMicroelectronics\hardware\stm32\2.0.0\libraries\SrcWrapper
"C:\\Users\\William\\AppData\\Local\\Arduino15\\packages\\STMicroelectronics\\tools\\xpack-arm-none-eabi-gcc\\9.3.1-1.3/bin/arm-none-eabi-size" -A "C:\\Users\\William\\AppData\\Local\\Temp\\arduino_build_323229/Blinktesrt.ino.elf"
Sketch uses 0 bytes (0%) of program storage space. Maximum is 1048576 bytes.
Global variables use 0 bytes (0%) of dynamic memory, leaving 98304 bytes for local variables. Maximum is 98304 bytes.
An error occurred while uploading the sketch
I'd probably try changing the board type in the IDE to STM32F103C8T6 (Blue Pill) which is much less exotic that the Nucleo-64 L476RGand use just the verify in the IDE (don't attempt to upload).
If it fails to verify with the STM32F103C8T6 board then it is likely to be an installation problem of the STM32 core. But a compatibility problem with your anti-virus program is also still possible.
Clear out that core in the Board Manager, switch off your anti-virus program, do the installation again and then try the verify/upload operation with your Nucleo-64 L476RG.