The reason behind "concieve": a common spelling misstep

What kind of error is it?

It is a spelling error.

Why do people make this mistake?

The incorrect spelling "concieve" results from applying the "I before E except after C" rule, but this rule has many exceptions, and "conceive" is one of them.

What is correct?

The correct spelling is "conceive". "Conceive" means to form or devise a plan or idea in the mind, or, in another context, to become pregnant.