Given the geometric series, find the least number of n so that the difference between the Sum to infinity and the Sum to n is less than

This is my take:

Substituting r and a into the formula of Sum to infinity and Sum to n,

Then, I decided to use logarithms but when I typed it onto the calculator, I found out that it was not possible (maths error) because = undefined. Anyone know where I had gone wrong? Thanks.