Please make sure that the 12-month period that you have selected is correct. It must include the time period when you incurred the expenses. Ex: if most of your expenses are from the end of 2018, then they will not be claimed if you choose January 2019 to December 2019 as your claim period.
Also, you can claim the total amount of eligible medical expenses minus $2,352 or 3% of your net income, whichever amount is smaller. So if your income is too low, then you might not be able to claim medical expenses for the year. You may be able to claim some or all of these expenses next year, though.
Finally, the medical expense tax credit is a non-refundable tax credit that you can only use to reduce the tax that you may have to pay. So if you have no taxes owning, you won't get the credit.