Thank you all for your comments.
I re-installed Ubuntu, thinking that it was a problem with my installation, but I get the same errors again.
Marcus, I re-installed git, but I get the same error.
I have no idea what is happening. I followed this steps
1 - Installed fresh Ubuntu 18.04.4 LTS
2- Installed git, cmake, swig, gnuradio.
To install gnuradio, I used:
$ sudo add-apt-repository ppa:gnuradio/gnuradio-releases
$ sudo apt-get update
$ sudo apt-get install gnuradio
3. Then when I use gr_modtool, I get the errors I reported in the first message.
Any help would be greatly appreciate it, thanks!