"Mkasi" is the singular noun meaning one pair of scissors, while "mikasi" is the plural noun meaning more than one pair of scissors (MI- class noun). So, "wa-" is not the correct way to form the plural.
But note that the English sentence is ambiguous as to how many scissors are meant, since the word always ends in "s." The most likely meaning would be one pair of scissors, for which reason it is translated here as the singular "mkasi" (although the plural "mikasi" should also be accepted).
"Makasi" is an alternate spelling for the plural "mikasi," although it is marked as an "uncommon and obsolete" variant spelling in some sources. "Mkasi" is indeed a correct singular form. Confirmed in the Swahili Practical Dictionary by Awde, Wiktionary (https://en.wiktionary.org/wiki/mkasi and https://en.wiktionary.org/wiki/makasi#Swahili), Glosbe (https://glosbe.com/sw/en/mkasi and https://glosbe.com/sw/en/mikasi), and several other dictionaries.