Question: numerically solved one non-linear equation in presence of units

Hi, again, I am a physist. It is convenient to solve problems in presence of units. I have a equation (1-variable) including a erf function to solve. As I substute all the constants (with units) and then use 'solve'. It returns a warning without any result. I am not really familiar with the numeric package in maple. Could anyone give me some advice? Thank you.

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