I entered my vehicle mileage and business mileage but no deduction is given. Do I need to complete something else for this to work?

Have you entered your Self-Employment income yet?

Yes, everything else for income/expenses is filled out, but I am not getting any deduction for the mileage, and when I click the link that says "Why didn't I get this?", an error page comes up on saying "Something went wrong..."

It turns out this a bug in the system currently.  You have to enter the year that the vehicle was put in use as 2021 in order for it to calculate the deduction.... Not sure if anyone is actually working on the issue or not, but if you change the date then all of the calculations are completed as expected.