You appear to be missing the \ at the end of the continuation lines here
[Service]
Type=notify
ExecStartPre=/bin/mkdir -p %h/mnt/%i
ExecStart= \
/usr/bin/rclone mount \
--fast-list \
--vfs-cache-mode writes \
--vfs-cache-max-size 100M \
%i: %h/mnt/%i