TechNormie Under settings>system>multiple users the bar isn't turned on. I assume that means only 1 user/profile?
Yes that will mean you are only using the Owner profile. However from reading your initial post again, are you stating that you have somehow uninstalled the default SMS app?
TechNormie I'd like to reinstall the default messaging app but don't know how
It shouldn't be possible as the only option in the default SMS app 'App Info' pane is to 'Disable it'.
It is highly inadvisable to disable or interfere with any system apps.
Check SETTINGS>APPS>SEE ALL APPS>MESSAGING and see if you have it disabled, if so reenable it.
Do the same for any stock app you may have tampered with and restore to default settings. If you continue to struggle it may be advisable to do a backup SETTINGS>SYSTEM>BACKUP on a USB drive and Factory Reset the device.