Write the formula and name of compound formed by the following combinations of ions: Cu+ and S2-?

Formula : Cu₂S

Trivial name : cuprous sulphide
Name in complete Stock system of nomenclature : copper(I) sulfide(II) / copper(I) sulphide(II)
Name in semi-Stock system of nomenclature : copper(I) sulfide / copper(I) sulphide

Your teacher wants you to answer with copper(I) sulfide, Cu2S, so that the sum of the "charges" equals zero.

The catch is that the entire premise of the question is a work of fiction. Cu^+ ions are not found in aqueous solution. Cu^+ will undergo disproportionation to form Cu^2+ ions and copper metal. Neither are there S^2- ions in solution. They will immediately react with water to make HS^- and OH^- ions.
S^2- + HOH(l) --> HS^- + OH^-

It is possible to form Cu2S by reacting powdered copper metal with sulfur gas in the absence of oxygen.
2Cu(s) + S(g) --> Cu2S(s) ........... but it is not a reaction of ions. Cu2S is not an "ionic" compound since the bonds have only about 10% ionic character. Instead the bonds in the network solid have high covalent character.

It is unfortunate that teachers often give overly simplistic questions that completely distort the chemistry which actually occurs, and do students a disservice in the process. It would have been better to use two aqueous ions which actually do react to form an insoluble product. Simply changing Cu^+ to Ag+ and using sulfate ion would give a simple reaction with an insoluble product where you had to add a subscript.
2Ag^+ + SO4^2- --> Ag2SO4(s) ......... silver sulfate.

Cu2S