That sentence has a subject: "a pearl". It's "on the ground" which is the copula's predicative.
When a sentence starts with a locative adverbial phrase, the dominant word order in Modern English is location-subject-verb. But the dominant word order of Old English, location-verb-subject, is still occasionally used.
But most people would indeed rather say "There's a pearl on the ground", "A pearl's on the ground", or perhaps "On the ground, there's a pearl".
If Duo doesn't accept it, the reason is because eventually there will be a "reverse tree" i.e. an English for Finnish speakers course. We would want to teach them to say "There is a pearl on the ground," rather than the literal translation. Imo we should not enter phrases that nobody would ever say.
What kind of pearl would be on the ground? I have not seen or heard that expression in the past 5 years of studying languages to Duolingo challenge. The unique native cultures of the translators is one of the things that makes Duolingo such an awesome way to learn a language...or three.