If you are using the free trial version of Vidnami you will see a watermark on your videos. The behavior of ths program is fully intentional. The watermark is added when exporting movies in order not to use the trial version of Vidnami for commercial purposes, but only for testing. This solution was introduced in mid-2020 due to a large number of frauds.
Of course, the watermark only appears on movies exported from the trial version. This version has been specially developed to allow users to familiarize with the application and to try out its features, and not for commercial work. The full version of Vidnami does not have these restrictions.
More about limitations of Vidnami free trial version you can read in Vidnami Free Trial Limitations post.