All working except admin???

I have installed 4.3.4 on a unix server with PHP8.1.  Database connection okay.  All working fine except when I attempt to access admin and get the following response:

Couldn't include '/home/sites/XXREDACTEDXX/public_html/vendor/autoload.php'. Did you run `composer install`?

(obviously I redacted the full URL here for security).

Has anyone got any ideas why it is happening and what I can do to fix it?  It appears all elements of the install went well and all the files I expect to see are in the right places with the right contents and permissions.  

Thanks in advance