What are the two errors ...

Less calories then the leading brand

Fewer calories than the leading brand

Explanation: no s at the end of brand

The two errors are Less instead of Fewer, and then instead of than.

The correct sentence is Fewer calories than the leading brands.

Explanation:

Fewer, since calories is a countable noun, for a small quantity with countable nouns we use fewer as a comparative. The sentence is a comparative, the necessary words to make this is than.