you can earn by exporting a project on Android with a free version of unity?
If you mean if you can use Unity’s free version for commercial projects, then the answer is yes. Yes you can.
Directly from the unity answers:
" It is all in the EULA.
If your company has not made more than $100000 turnover in the last fiscal year you can publish and sell without pro license.
If you have made that amount you can publish and sell but you need the Pro license.
I would think the second case also applies if you are not selling but I would also think you are in the first case."