ffmpeg batch overlay Error: "Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_overlay_0"
Looks like you forgot -i. Try:

@echo off
REM Add watermark to video batch
for %%a in (*.mov) do ( ffmpeg -i "%%a" -i watermark.png -filter_complex "overlay=10:10" "output\%%~na.mov"
)
pause