Lohr,
The objcopy line is for some unknown reason missing the executable file - .elf file as the input parameter.
You should see:
arm-none-eabi-objcopy -O srec My.elf "my.srec"

elf file is specified by variable $(EXECUTABLES)
Which is automatically generated within makefile:

If this variable is empty on your side i'd recommend you to delete entire output directory e.g. "Debug" and rebuild the project.
The makefile will be regenerated again.
Another check is to create a new empty project and enable Create Flash image.Build the project - does it generate srec?
Thanks!
Stan