After installing a module, you have to wait for the toast to expire before you can install another.
Thanks, I already raged about this (not the only one) and the design/web guys already have a ticket for this.
As a workaround, you can immediately reload the page to dismiss the toast and expose the restart prompt.
It would be nice if you could multiselect and install multiple modules at once.
We have that planned for an early release of 8.3, but not for 8.3.0. Just not enough time to fit everything in.