If you're being prevented from changing your marital status in TurboTax, it is likely due to how you've chosen to prepare your return.
For example, if you initially chose to prepare your return together with a spouse/partner, TurboTax will not allow you to change your marital status to single (as single is not a valid status for someone who has a spouse/partner).
Additionally, if you've transferred your return over from a previous year (and you were something other than single), TurboTax will not allow you to change your marital status back to single. According to CRA rules, once someone has been something other than single, they can't return to single status again (read the Marital status page as there are other options available).
If neither of these situations apply and you're unable to change your marital status, you need to start a new return.