MgBr2 ….. Use the oxidation states, instead, +2 for Mg, and -1 for Br.
Lewis structures are for discrete molecules. MgBr2 is not molecular, but exists as a network solid with alternating Mg and Br atoms in three dimensions.
Your teacher wants you to say this.
Br• ….•Mg• …. •Br
Each Br has three lone pairs.
The two electrons from Mg are supposed to be transferred to Br to give each an octet of electrons and a -1 charge, while Mg has a +2 charge.
The catch is that there isn’t a complete transfer of electrons. A bond between Mg and Br has only about 50% ionic character. Therefore, there is a significant amount of “electron sharing” taking place, too.