The only time periods that you can use with Roadmaps are Annual ones.
When you set up a Roadmap, it checks your fiscal periods and only Annual periods are used.
The start period and duration are all based on which annual timeperiods are set up.
If you only have Annual periods for 2017, 2018, 2019, and 2020, then the Roadmap can only start on one of these 4 periods and the duration is constrained by the remaining periods.
For example, if you select 2018 as your start period, then the max Roadmap duration will be three years (2018, 2019, and 2020).
If the start period is set to 2020, then the max duration is one year, 2020.
If you do not want to have yearly time periods available for the Classic UI, you can create them and then leave them set as inactive.
The New UI uses inactive time periods.