The displayed message is not cause by Nextcloud but your Apache web server. It tells you that the mentioned PHP library cannot be loaded because of an undefined symbol. This kind of problem appears if e.g. the pdo_mysql module has been compiled in a different environment or requirement couldn’t be found on exists in a wrong version. Wrong version means, that e.g. a required library exists on your system, but with in wrong version etc.
Aunt G. provides more detailed information to fix the problem:
https://www.google.com/search?ei=8RjZXPDFIc3ZwQKfoZOYCA&q=pdo_mysql.so+undefined+symbol+mysqlnd_allocator+in+unknown+on+line+0&oq=pdo_mysql.so%3A+undefined+symbol%3A+mysqlnd_allocator+&gs_l=psy-ab.1.0.0i19j0i22i30i19.23016.23016..27629...0.0..0.63.63.1......0....1..gws-wiz._jsKvhVs7uI