why is this wrong

Question

I’ve looked at the forum responses and still don’t understand why ‘je suis dix minutes en retard’ was marked as incorrect. More clarification please.

Accepted Answer

Hi Richard,
Thank you for your question. Although it is more correct to use the other two expressions we suggest, you might hear what you propose colloquially in France, so we have decided to add it to the possibilities.
Hard to justify why it would be wrong, rather than just not what you are likely to hear, and as the other expressions are harder to construct (being different from the English) it is important to learn the way most people will express the notion of 'being 10 minutes late' in the correct way.
Hope this helps!

Answer

Bonjour Richard,
If you had written:    "J'ai dix minutes de retard" this would have been correct.
Using être you are expressing "a state" and the "state" is that of lateness (en retard)
So the sentence needs to be written "Je suis en retard" followed by the extent of the lateness, in this case 10 minutes. "Je suis en retard de dix minutes" --> I am late (state) by ten minutes (extent of lateness)
https://progress.lawlessfrench.com/revision/grammar/how-to-express-being-early-or-late-precisely-in-french
Hope this helps.
Bonne continuation
Jim