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When I googled parallax error a lot of information comes about the rangefinder cameras' viewfinders' parallax error. I also googled "parallax error panorama" but a few information comes. Now I want to know that 'is this the parallax error?'
(this-the grid on floor is distorted...)
I also want to know how it happens...
No, that is not parallax error, that's remapping distortion and is perfectly normal for a panorama that wide. When you have an image on a flat plane, you can't actually get more than a 90º field of view, without some distortion. When you do scene coverage that's wider than that, then the image has to be remapped to a flat plane from the spherical view. That generally results in curved lines.
You can change the curvature of the lines by changing the output projection type (e.g., from cylindrical to Pannini). However, some simple stitcher packages don't allow you much choice between projections. And no matter what projection you choose, that curvature will still be there to some degree.
Parallax is something different. To quote from Wikipedia:
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.
In the case of a rangefinder camera, the difference in viewpoint between the viewfinder and the lens you use to take the image introduces parallax so what you see through the viewfinder may not be the image you get through the taking lens. That difference is relatively minimal for farther distances, but becomes much more major when used at close distances.
When shooting a panorama, if you rotate the lens around a point other than its no-parallax point, you've essentially moved the lens between member shots and created parallax. Parallax error means you get breaks in lines in a stitched panorama where the shifting viewpoint put the line in a different relative position between shots.
See also:
We've known about this effect and been manipulating it for years.
This is a good example of perspective, specifically, spherical perspective.
Here's a good explanation using diagrams that would be difficult to illustrate using only photography. It is not parallax. It is a curvilinear effect caused by your perception of the panorama image.
