spelling

The error in this example is not related to spelling -- "barn" is the correct spelling. This is an adverbial clause answering the question "when". It requires a main clause to go with it in order to form a grammatical sentence.

For instance:

"You could do something else while Lennie plays with the new puppies in the barn."

"While Lennie plays with the new puppies in the barn, you could do something else."

In this case, the software you used has incorrectly identified "barn" as a spelling error and failed to spot the actual error.

Every month or two, somebody complains about this problem. Grammarly tells you barn is misspelled.  And, for some reason, 9 times out of 10, the problem is with barn and not some other word.

You need to contact Grammarly Support -- there is a link at the bottom of this page -- and complain. In the past, Grammarly Support has told me the problem stems from a bizarre interaction between your browser's cache and the server software. I have found that if clear your browser's cache completely, the problem goes away for a while.

Meanwhile, Peter is right. You have a sentence fragment. You need to attach it to a main clause.I hope this helps.