Hi,

You can achieve it using python constrains. See more about constrains here: https://www.youtube.com/watch?v=50cecNF3OyQ

In your case, you can add a constrain as follows in your main model:

@api.constrains('line_ids')
def _check_lines(self):
    for record in self:
        if not record.line_ids:
            raise ValidationError(_("Warning Message!."))

Update line_ids with your o2m field name and validation message as per your use case.

Thanks