Singular in Jupyter notebook

I installed the the Singular kernel for Jupyter and can write singular in a Jupyter notebook. That is wonderfull. But when I tried to reopen it: error

TypeError: ext.toLowerCase is not a function

it was not possible to view and edit my work. Maybe it is linked with the fact that I can't install PySingular with pip3:

Processing ./PySingular-0.9.7.tar.gz
ERROR: Command errored out with exit status 1:
command: /usr/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-req-build-q92swtcy/setup.py'"'"'; __file__='"'"'/tmp/pip-req-build-q92swtcy/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-req-build-q92swtcy/pip-egg-info
cwd: /tmp/pip-req-build-q92swtcy/
Complete output (5 lines):
Package singular was not found in the pkg-config search path.
Perhaps you should add the directory containing `singular.pc'
to the PKG_CONFIG_PATH environment variable
No package 'singular' found
Could not find a suitable libsingular configuration. Do you have libsingular headers installed?
----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

Does someone can help me?

Thanks

I found a solution: before saving I change the kernel (for example Python). The I can reload it and changing the kernel to execute it.

How did you install the singular kernel in Jupyter?