Hello Leandro,
Welcome to the forum! You cannot register .com domains with mixed scripts. See
this Verisign document, especially Section 3. If you dig more deeply into the document, you can find some exceptions. For instance, you can mix Chinese characters with Latin.
The reason for the prohibition, as you might have guessed, is to prevent phishing. If you owned ebаy.com, where the third letter is the Cyrillic а, not the Latin a, you could confuse people.
Some of the country registries (.kz, .uz, and perhaps .ws) may not enforce this rule as strictly, though in that case you probably have to register your name as punycode.
By the way, the members of this forum specialize in registering, buying, and selling generic-word domains, rather than company names, so hopefully no one would register your company's name and try to sell it back to you. But in any case, since your company name is mixed Latin and Greek, it can't be registered in .com.
Avtal