Pointing out customer's mistake politely in customer support setting [closed]

If someone sends in a support request with a mistake that they clearly made, what would be the polite way to answer?

I initially used the term

It seems that you have forgotten to fill in this and that

but others pointed out it should be started with

You should fill in this and that

In Estonian, which is my native language, using the first is considered more polite, whereas second is seemed more like a command making it a rather harsh way.

Thank you.

Politely solving a problem arising from a customer error

Your method is more polite than your colleagues', because it is less blunt.

Even more polite would be to omit reference to the customer's failure to do something. How about:

"Please fill in this and that."

"You can achieve this by filling in this and that."

"It looks like this and that are not filled in. Could you fill them in please?"

"For the X process to work, it needs Y and Z. Please fill them in: it should then work."

That way you de-emphasise the existence of a mistake, or depersonalise it.

An intelligent customer would recognise that you have seen he has made a mistake, and that you are polite about pointing it out, and will form a favourable view of your company's customer handling.

An unintelligent customer would not notice, so nothing is lost.