A requirement for a K-1 fiancé visa is that both you and your fiancé are free to marry.
In order to be legally free to marry, any prior marriage needs to have ended in annulment, divorce, or death.
If your fiancé is still married, then you cannot apply for the K-1 fiancé visa. If you, apply for the K-1 fiancé visa while one or both of you are not free to marry, then your case will be denied. This is true even if a divorce decree is obtained while your case is still pending.
Ultimately, prematurely filing a case will have the unintended consequence of having your fiancé arrive in the United States later than expected rather than sooner. Hence, you should not file your K-1 fiancé visa case until both you and your fiancé are free to marry.
Family and divorce law can be very complex. The difficulty and time required to obtain a divorce varies by country and even by state. Many individuals believe that they are divorced when their divorce is not yet final. A California study a few years back determined that over 20% of the people who thought they were divorced had not even completed all the paperwork needed to finalize their divorce. Hence, they were legally still married. It is advisable to consult with a family law attorney to ensure that any prior divorces or annulments are actually final. The attorney will likely need to review any applicable divorce, annulment, or death documents.
Disclaimer: The information provided on this site is not legal advice but general information only. This article is only a very basic introduction to this topic. You should seek a competent Immigration Attorney to review your specific facts and circumstances before proceeding with your case.