Assuming you purchased from Vmware, you should be able to log into your account at Vmware and check your license. Otherwise, contact Vmware sales. This is a user helping user forum, not an official Vmware support site. 

Try to enter the serial with regedit instead.

Not sure regedit works on Ubuntu?

version 18.04 is a little dated. 

On Ubuntu Linux? Seriuosly? Thank you for spending your time trying to help me, by the way...

LOL - sometimes reading the full post helps .... sorry.
Have you tried
sudo /usr/lib/vmware/bin/vmware-enter-serial

Ulli

Thank you again Ulli, I tried, but got a segmentation error. Greetings, Alessandro

Try
/usr/lib/vmware/bin/vmware-vmx --new-sn ABCDE-FGHIJ-KLMNP-QRSTU-VWXYZ

That seems to work on Debian 10

If that still does not work close Workstation completely and try to edit
/etc/vmware/license-ws-160-e4-202001
The file should exist but the line
SERIAL = "12345-12345-12345-12345-12345"
may be missing.

Ulli

Thank you again Ulli, tried bot suggestion, I got no errors, but Workstation Pro refuses to run.

What does refuse to run mean ?
Do you have files named license-ws-* in /etc/vmware
Which exact version do you try to run ?

I got stuck into the license activation panel, with a message "Invalid activation key". No way to step further.

# vmware-installer -l
Product Name Product Version
==================== ====================
vmware-workstation 16.2.3.19376536

(purchase date May, 17, 2022)

# lsb_release -a
LSB Version: core-9.20170808ubuntu1-noarch:printing-9.20170808ubuntu1-noarch:security-9.20170808ubuntu1-noarch
Distributor ID: Ubuntu
Description: Ubuntu 18.04.6 LTS
Release: 18.04
Codename: bionic

# uname -a
Linux otto 5.4.0-110-generic #124~18.04.1-Ubuntu SMP Fri Apr 22 12:01:21 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Hardware is an 8 cores, 32 GB RAM, 32 GB swap, 1 TB SSD disk, Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz

Player (free version) works perfectly, but I'm planning to use VMware suite for work, so I'm trying to upgrade to Workstation pro 16.x ...

I'll try contacting sales representatives, just in case something went wrong with my license activation key VMware sites today are experiencing some problems and service outages.

Thank you very much again for your very kind help and the time you spent on my problem, ciao from Italy,

Alessandro

Please list the files you have in
/usr/lib/vmware/licenses/site/
and in
/etc/vmware

Ulli

Thank you Ulli and all who helped me! Today my problem seems solved: licence activation code and input panel worked. I can not write a solution since I've done nothing different today, so I suspect some network or remote server problems during licence activation.  Greetings from Italy!