Check Let Start11 enhance the taskbar on the Taskbar tab first. On Windows 11 that switch is what unlocks the per-display taskbar options, and Start11 settings greyed out for a second monitor are usually it being off.
Our documentation is explicit about this for the settings next door: "To further configure the start button and/or taskbar items to left aligned or center aligned on Windows 11, first enable the 'Let Start11 enhance the taskbar' setting from the Taskbar tab. This will also allow the options to hide the start button on primary and secondary displays."
Side placement is also newer and narrower than top and bottom. Vertical taskbar support came in with 2.5 as an experimental feature and it needs Windows 11 23H2 or newer. If the enhanced taskbar switch does not bring left and right back for the external display, post your Windows build here and we can work out whether that is a limit or a bug.
One thing to expect if you do get a side taskbar running on two displays. Where the external monitor sits to the left of or above the laptop screen, a taskbar on the left, right or top has been reported to leave desktop icons not painting on the desktop. That is cosmetic, the files stay exactly where they are.