How to know?

How to know if there is an error in ubuntu OS?..
or is there any other way to see those errors?.. and if there was/is how do fix it?..

Well it depends on what you mean. Why would you think there is an error? Have you done something to your system in order for there to be an error, for example, install a new program but it wasn’t compatible with the version of your Ubuntu install, or perhaps you installed some 3rd party drivers for your graphics card, but it didn’t go as planned.

We need more details in order to help you.

If there was an error, Ubuntu would tell you, unlike Windows.