Who authored the Targum on Kohelet? I was unable to find an answer online.
The identity of the author remains a mystery, but Pinchas Churgin analyzed the targum in his book תרגום כתובים and wrote that this much could be concluded about the author:
The targum was composed in Eretz Yisrael, sometime after the closing of the Yerushalmi and Bavli Talmuds. The author decries the harshness of the exile and the cruelty of the surrounding nations, which was the situation during the Roman rule over Am Yisrael. There are no hints of Arabic in the targum, but there is a lot of Greek and Latin - therefore, the author lived in Eretz Yisrael shortly before the Arabic conquest of the land in the second half of the 7th century.