https://www.linuxbabe.com/linux-server/how-to-enable-etcrc-local-with-systemd

The solution As you can see from above, The unit file have no [Install] section. As such Systemd can not enable it. First we need to create a file:

sudo nano /etc/systemd/system/rc-local.service
Then add the following content to it.

[Unit]
 Description=/etc/rc.local Compatibility
 ConditionPathExists=/etc/rc.local

[Service]
 Type=forking
 ExecStart=/etc/rc.local start
 TimeoutSec=0
 StandardOutput=tty
 RemainAfterExit=yes
 SysVStartPriority=99

[Install]
 WantedBy=multi-user.target

Save and close the file. To save a file in Nano text editor, press Ctrl+O, then press Enter to confirm. To exit the file, Press Ctrl+X. Next, run the following command to make sure /etc/rc.local file is executable.