Final answer:
The error in the sentence is that it is a run-on sentence and needs a period or semicolon to separate the two complete thoughts.
Step-by-step explanation:
The error in the sentence is that it is a run-on sentence and needs a period or semicolon to separate the two complete thoughts. To fix the sentence, you can add a period or a semicolon between 'year' and 'this'. Here's the corrected sentence:
We adopted a kitten last year. This year, we got a dog.