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It doesn't allow me to fix and I don't know how am I supposed to fix it.
(it is strain, so, it is unitless, or in/in is also fine)
The symbolic solver does not handle units the way you could expect; at best it treats them like undefined variables.
If you are after numeric results only, you should consider using the root function.
You may follow the symbolic evaluation (->) immedeatly by a numeric one (simple = ) to get the numeric result you expect.