Using variable in command path for ExecStart in systemd service

You can't use variables in the actual command. systemd.service:

The command to execute must be an absolute path name. It may contain spaces, but control characters are not allowed.

You might wan't to wrap it in a shell command (which does parameter expansion):

ExecStart=/bin/bash -c '/${TESTEXTSERVICESFILES}/syslog/bin/nxlog -c ${TESTCONFDATA}/syslog/nxlog.conf'

Instead of Environment=, use EnvironmentFile=, to define multiple environment variables.

[Service]
Type=simple
EnvironmentFile=/etc/sysconfig/customsyslog
ExecStart=/bin/echo ${TESTEXTSERVICESFILES}/syslog/bin/nxlog $TESTCONFDATA

/etc/sysconfig/customsyslog would contain key=value pair, e.g.,

TESTEXTSERVICESFILES=/opt/test/extservices
TESTCONFDATA=/storage/test/conf

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