Can't make animated gif with transparent background in Photo Paint Home & Student 2019


In Photo Paint Home & Student 2019, I try to make an animated gif with transparent background, but can't succeed.

When I save or export the movie, in the GIF98 animation options menu, the select transparancy color button doesn't function, though it's not greyed out.

Another try: when I try to set the number of the transparancy color to 127 for example, it doesn't result in any transparency.

Am I missing something? Or is it a bug?

Thank you!