Learn Arabic (Modern Standard): يا فاطمة، انتظري في الكواليس!

Because it is a proper noun being directly addressed. In Arabic, a proper noun in the vocative (after يا) is built on the nominative sign, which means it takes a single damma (فاطمةُ) and no tanween.

Why is the command انتظري instead of انتظر?

The speaker is addressing Fatima, so the imperative verb must be in the feminine singular form. This is done by adding the suffix ـي (yā' al-mukhaṭabah) to the base Form VIII command انتظر.

The word الكواليس for backstage looks like a broken plural. What is its singular form?

The singular is كوليس. It is a loanword from the French theater term 'coulisse'. In Arabic, it is treated as a standard noun and takes the broken plural pattern كواليس (similar to words like مفاتيح).

How should the transition between فاطمة and انتظري be pronounced?

The first letter of انتظري (a Form VIII imperative) has a connecting hamza (hamzat wasl). This means its initial vowel sound drops out in connected speech. You connect the damma of فاطمةُ directly to the 'n' sound, pronouncing it as 'yā Fāṭimatun-taẓirī'.