Error Code: 1248 Alias needed in MySQL
You need to use JOIN by ID instead of the UNION.
Using JOIN there is no need of the sub-query.
Also ( before SELECT is not required after the INSERT INTO.

So the working query is:

INSERT INTO Company (Year, Company, Revenue) 
SELECT FR.Year, FR.Company, CF.Revenue
FROM Fortune500 FR
JOIN Comp_Financials CF ON CF.ID = FR.ID;


Added More explanation in this post, why you need to avoid UNION, and use JOIN for your requirement.