sage 9.1 installation problem

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I have downloaded sage-9.1-Ubuntu_18.04-x86_64.tar.bz2 binary into a folder named Sage 9.1. Then from that location on terminal I typed:

./sage

A patching sequence started with the final mesagge:

Error: The path to the Sage directory ($SAGE_ROOT) MUST NOT contain spaces.
It is currently "/home/javi/Sage 9.1/SageMath".
Please correct this by moving Sage (or renaming one or more directories) first.
Exiting now...
Error setting environment variables by sourcing '/home/javi/Sage 9.1/SageMath/src/bin/sage-env';
possibly contact sage-devel (see http://groups.google.com/group/sage-devel).

Then I thought "this must be fixed by changing the directory name"...wroooong. I changed the path to /home/javi/sage/sage-9.1 (changed "Sage 9.1" for "sage" and "SageMath" for "sage-9.1"). Then run again

./sage

and gave me:

It seems that you are attempting to run Sage from an unpacked source
tarball, but you have not compiled it yet (or maybe the build has not
finished). You should run `make` in the Sage root directory first.
If you did not intend to build Sage from source, you should download
a binary tarball instead. Read README.txt for more information.

Aditionally I have:

$python --version
Python 3.7.7

Would anybody help me? Thank you in advance.