Isn't the imperative of chiamare "chiama?"
Yes, the informal is "Chiama!".
The courtesy form is "Chiami!"
If "Chiama" is not accepted, you should report it. :)
Sometimes it helps to look at the verb conjugations for clarity when one gets confused. This web site may help. Look at the Imperativo.
Why isn't you call security accepted
Duolingo uses the exclamation mark to indicate the imperative mood. If you remember that you will have a lot fewer answers dinged.
should be accepted
The tu and lei forms swap places in "are" imperative. Use the above site to compare the imperative in "are", "ere" and "ire" forms (http://www.coniugazione.it/verbo/chiamare.php)
why 'call THE security' not accepted?