I am trying to itemize my medical expense but the system won't let me and keep showing me this message: right now, the standard did save you the most taxes.

It is telling you that you do not have enough to itemize and the standard deduction gives a better tax result.

On medical to itemize your medical expenses including insurance premiums must exceed 7 1/2% of your AGI.   Only the amount above the7 1/2%  is deductible.     

All of your Schedule A deductions must exceed the standard deduction.  TurboTax is going to take whichever outcome gives the better result.   Standard Deductions for 2021 are:

• Single/Married Filing Separate - $12,550
• Married Filing Joint                   - $25,100
• Head of Household                   - $18,800

Take a look at where your Schedule A (itemized deductions) are totaling up to, it may not be worth continuing to enter information if you are not going to exceed the standard deduction.