Slope-intercept form writes a line as $y = mx + b$, where $m$ is the slope and $b$ is the y-intercept.

The slope comes from rise over run between two points, and the intercept is the point where the line crosses the y-axis.

If the two points share the same x-value, the line is vertical and cannot be written in slope-intercept form. In that case the equation is $x = c$ instead.

The calculator warns about this because the slope would be undefined.