you can't continue because there is no empty bar. make one (or more, it's better
) and then try again.
press
B while holding
ctrl. a new, empty bar will be added at the end of score. now you should be able to continue.
maybe it will be continued in a same row (system)... later you can mark last barline that you want to be in that row (click on it), and press enter. all bars behind marked barline are now moved in next row.