pi@pi4:~/NetXMS/netxms-3.0.2258/contrib/startup/systemd $ ls
Makefile  Makefile.am  Makefile.in  netxmsd.in  nxagentd.in  README

do not include the build service file after the make.

While waiting for a maintained RPi packages, im building agent from source on RPi.
Using something like this to get the latest version built:

sudo apt-get install -y links libssl-dev build-essential
MAJOR=$(links -dump "https://www.netxms.org/download/releases/" | egrep "([0-9]+\.[0-9])"  | awk '{print $1}' | cut -d "/" -f1 | tail -n1)
MINOR=$(links -dump "https://netxms.org/download/releases/$MAJOR" | sed 's/[^0-9.]*\([0-9.]*\).*/\1/' | egrep -v "^$|^\." | sort -t. -k3,3n | tail -n1 | sed 's/\.$//')

echo "Create build folder"
mkdir ~/NetXMS
cd ~/NetXMS
wget --no-check-certificate https://www.netxms.org/download/releases/${MAJOR}/netxms-${MINOR}.tar.gz

echo "Unpack source to clean folder"
if [ -d netxms-${MINOR} ]; then rm -rf netxms-${MINOR}; fi
tar -zxvf netxms-${MINOR}.tar.gz
cd netxms-${MINOR}

./configure --with-agent --without-curl --disable-ssh --disable-mqtt --prefix=/opt/nxagent
make
RETURN_CODE=$?

if [ $RETURN_CODE -gt 0 ]; then
    echo "Something bad happen: $RETURN_CODE"
    exit
fi

echo "Stop current"
sudo systemctl stop nxagentd

echo "Make install"
sudo make install
RETURN_CODE=$?

if [ $RETURN_CODE -gt 0 ]; then
    echo "Something bad happen: $RETURN_CODE"
    exit
fi

sudo cp contrib/startup/systemd/nxagentd.service /etc/systemd/system/
sudo systemctl enable nxagentd