Learn Croatian: U restoranu nema struje. - There is no electricity in the restaurant.

Because it comes after nema (there is no). In Croatian, when you state that something is missing or doesn't exist using nema, the missing thing must be in the genitive case. Struja is a feminine noun ending in -a, so its genitive form ends in -e.

What verb does nema come from?

Nema is the negative form of ima (has / there is). It merges ne (not) and ima. While it literally means does not have, it is the standard Croatian way to say there is no or there isn't.

Why is an -u added to the end of restoran?

The preposition u (in) tells us we are talking about a location. This means the noun following it must be put into the locative case. For a masculine noun like restoran, you form the locative by adding an -u to the end, giving us u restoranu.

Could I change the word order and say Nema struje u restoranu instead?

Yes, you absolutely can. Croatian word order is very flexible. Both versions are correct, but starting with u restoranu just puts a little more focus on the location.