Hi Pasquale,

the short answer: Linux Mint 21.3 is not a supported distribution, as per the Linux Desktop Client Support Matrix.

a longer version: The Linux client has not been developed or updated for a long time. The support matrix shows "Ubuntu 16.04.4 LTS". The install script only checks for "ubuntu", though. The Line giving you the error is

if [ $linux_distribution = "Debian" ] || [ $linux_distribution = "Ubuntu" ]

and $linux_distribution ist empty, so the test command does not get a unary operator, twice - leading to the double error message. The value is probably empty because it is set via code using "lsb_release -i" (in lines 56-74 of the script if you want to check). This has to contain "Ubuntu", "Debian" or "SUSE" for the script to work, and "lsb_release -i" gives only the distribution ID which usually does not include the base.

I am rather confident you may be able to get the client to run, but to be absolutely clear: you will have no support and it will install packages from March 2019 which can have compatibility problems with your system or which may introduce security issues.
That warning being given, if you still want to install it (e.g. for testing) you just have to duplicate and modify a few lines in the installscript (as above, in lines 56-74) so it returns "Ubuntu" as retval when the lsb_release output contains whatever Mint 21.3 outputs on "lsb_release -i". Just use the existing three values as reference.

HTH!

Christian