What are the two errors ...

Less calories then the leading 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.

Fewer calories than the leading brand

Explanation: no s at the end of brand