Sometimes, WordPress files can appear to just go missing with no clear reason. Usually, this is caused by an automatic update of WordPress which failed to complete.
If you installed WordPress with Softaculous, you can simply use Softaculous to update WordPress which should fix your files.
Alternatively, you can fix the WordPress files by hand:
@Oxy’s solution may also work, but the file he linked to is from an unreleased version of WordPress, which may not be compatible with the rest of the WordPress files currently installed. So it’s better to overwrite all files with the right versions to make sure all WordPress files are from the exact same WordPress version.