This is how my Synfig window looks like. How to turn animation toolbar on?
edit. I am not able to attach bigger picture! whn I’m pushing “Submit” to send this pot, something changes the size of the file so from 147kb it went to 53
try to extend the canvas duration
first go to canvas->Properties
Then go to tab Time and set the end time to other than zero
the Duration will follow it right away (by adding 1 extra frame based on the end time)
then save the changes
If I remember well, it happens when you open directly a SIF file created with Inkscape.
Instead try to create an empty Animation, then use Import function from the menu 