I am guessing a bit, based on your description.

Wolfram Mathematica, whether it is on your desktop computer or your laptop, has a detailed set of documentation on exactly what notation can be used with it.

Wolfram Alpha is a completely different product with a very different set of rules and, as far as I am aware, does not have a set of detailed documentation on exactly what notation can be used with it.

To complicate this, Wolfram Alpha does accept some Wolfram Mathematica notation and I have been able to use that to get it to solve problems that I was otherwise unable to solve. But there are many Wolfram Mathematica notations that Wolfram Alpha does not understand. And vice versa to a much greater extent.

To further complicate this, some people will post questions here about Wolfram Alpha and some readers will assume those are Wolfram Mathematica questions and give responses that cannot be successfully used in Wolfram Alpha.