Admittedly, it's not very clear what's going on because once you Save & Build, it looks like nothing has happened.
You need to scroll down..
PHP Compiler Started in Background. You can monitor PHP rebuild from the console using command: tail -f /var/log/php-rebuild.log
Go the the Dashboard and you should notice Load has risen, as well as cc1 showing in the process list. (This is not difficult stuff, guys but so many people need hand-holding these days
).
Appendix: PHP 7.3.16 switched successfully on CWP Pro VM - took 15 minutes.