Probably you (or your hoster) need to install/enable the PHP exif extension, see https://secure.php.net/manual/en/exif.setup.php

glenn wrote:

thanks, erAck!

The techs at Liquid Web installed it for me.

Unfortunately, the error message persists. I had them double check it and they say it's all good to go.

Any idea if there's some other PHP or Piwigo plugin or configuration necessary?

Piwigo uses PHP exif module. this need to be installed on your host, then you need to check if module is activated in PHP.ini, then you need to restart the PHP service.
Then you can check php info or you can make some little php files for tests

<?php
if (function_exists('exif_read_data')) {
    echo "php exif module is installed";
} else {
    echo "php exif module is NOT installed";
}
?>

and

<?php
$exif_data = exif_read_data('picture.jpg');
print_r($exif_data);
?>

is everything is fine go to Piwigo administration, Local files editor, search the lines where the exif configuration is.

The standard tool to read or modify EXIF data is the exfitool: https://owl.phy.queensu.ca/~phil/exiftool/

I don't think it will be possible to modify EXIF via Piwigo.