Hello Rebecca! Welcome to the FamilySearch Community!
This is a very good question. The answer is found in the General or Basic Indexing Guidelines or in the Additional Information section of most projects under the heading Corrected and Crossed Out Information.
Corrected and Crossed-Out Information
Here is a link to the Basic Indexing Guidelines. https://www.familysearch.org/help/helpcenter/article/how-do-i-use-familysearch-indexing-basic-indexing-guidelines
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While we may not know why a marriage license shows "Cancelled", there is likely some useful information located in this document that could be useful for learning more about our ancestors. It would be reasonable to try to confirm with other records if the marriage actually happened, as one would wonder if the couple decided to not get married. However, there may not be other records to find ages, dates of birth, or other information about the people included in this record. Some marriage records include information about the parents of the bride and groom that my not be found in other ways or other records.
If the indexing instructions for that project do not answer that question directly then I would go ahead and index it which will allow it to be found then the person finding it can evaluate its value.