As root, edit rc.local

vim /etc/rc.local

Add the following and whatever commands you want to start up on system boot:

#!/bin/bash

exit 0

Make it executable:

chmod a+x /etc/rc.local

vim /etc/systemd/system/rc-local.service

Add the following:

[Unit]
 Description=/etc/rc.local
 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

Enable the service:

systemctl enable rc-local

Start the service (keep in mind whatever you put there for startup will also start)

systemctl start rc-local.service

Check the status to confirm it worked:
systemctl status rc-local.service

It'll produce a similar output if all went well:

● rc-local.service - /etc/rc.local
 Loaded: loaded (/etc/systemd/system/rc-local.service; enabled; vendor preset: enabled)
 Drop-In: /usr/lib/systemd/system/rc-local.service.d
 └─debian.conf
 Active: active (exited) since Mon 2024-11-11 17:08:19 UTC; 5s ago
 Process: 37216 ExecStart=/etc/rc.local start (code=exited, status=0/SUCCESS)

Nov 11 17:08:19 server systemd[1]: Starting /etc/rc.local...
Nov 11 17:08:19 server systemd[1]: Started /etc/rc.local.