Replace decimal commas with decimal points, single backslashes with double backslashes, and \cdot with just a space:

ToExpression["\\sqrt{(\\frac{447.15  10^{-9}  0.87}{0.21 \
\\sqrt{(0.87)^2+(0.21)^2}})^2  (0.02)^2+( \\frac{447.15  \
10^{-9}}{\\sqrt{(0.87)^2+(0.21)^2}}-\\frac{447.15  10^{-9} \
\\sqrt{(0.87)^2+(0.21)^2}}{(0.21)^2})^2 (0.01)^2}", TeXForm, Hold]
ReleaseHold[%]

Check carefully the hold expression. You may need to insert extra braces {} in the input to get the correct interpretation.

let's say g = ... now solvable?

As Gianluca pointed out, that is not valid Wolfram code, decimal commas need to be replaced with decimal points etc. Check his answer, it gives the following

$$\sqrt{0.01^2 \left(\frac{1}{447.151^9 \sqrt{0.87^2+0.21^2}}-\frac{\sqrt{0.87^2+0.21^2}}{447.151^9 0.21^2}\right)^2+0.02^2 \left(\frac{0.87}{447.151^9 \left(0.21 \sqrt{0.87^2+0.21^2}\right)}\right)^2}$$

and corresponding WL code

Sqrt[(0.87/(447.151^9*(0.21*Sqrt[0.87^2 + 0.21^2])))^2*0.02^2 + 
   (1/(447.151^9*Sqrt[0.87^2 + 0.21^2]) - Sqrt[0.87^2 + 0.21^2]/(447.151^9*0.21^2))^2*0.01^2]

If that looks right then evaluate it. The result is

2.9798*10^-25