I am gonna try another software anyway as it seems quite complicated to me.
I know this thread is old, and presumably you've made your move to something else. But at least part of the reason you're finding weewx so complicated is that you're using a random five-year-old guide, written for people with a different hardware/software platform than you're using. And on top of that, it has an error which is likely causing the part where you're trying to set the MySQL password for the weewx user to not work--which would result in the error you're posting. Weewx has perfectly good documentation on its own web site, which likely would have gotten you up and running with minimal difficulty.
To be specific, the problem is with this command:
CREATE USER 'weewx'@'localhost' IDENTIFIED BY 'weewx.;
It should read:
CREATE USER weewx@localhost IDENTIFIED BY 'weewx';
But, as already said, you almost certainly don't want or need to be using MySQL anyway, for at least two reasons: (1) it further, and unnecessarily, complicates the setup, and (2) it will probably wear out the SD card very quickly, unless you're accessing MySQL on another host (which would be a better configuration, but would even further complicate the setup).
I see this a lot with FreeNAS as well, and it's equally baffling there. People would apparently prefer to follow some random blog post from three years ago than the actual manual (which, in the case of FreeNAS, is very thorough and up-to-date).