Due to a remote server change by Instagram, trying to post a carousel post will result in a failure during the final configuration step (displayed around 85% in Flume). With the method with which Flume is developed as an unofficial product, it is not possible to foresee changes that Instagram may make ahead of time such as this.
This issue is resolved for a future update of Flume, however that update cannot be released yet until every function of Flume is updated for compatibility with Instagram's latest infrastructure. This process is on-going, and unfortunately no exact release date can be provided at this stage (approximately end of Q1 2020).
If you purchased a Flume Pro licence after November 1st (the approximate date that Instagram made this server change), and cannot use Flume Pro effectively due to this issue (ie. you do not upload single photos or videos), you may request a refund. Please note that all refund requests have to be processed manually, however all refunds regardless of the usual 30-day refund window policy will be honoured.