Cannot Record Payment to a Contribution in Pending Pay Later Status (WordPress 5.0.10, CIVICRM 5.21.2)
thanks for your guidance, here is the PHP error and it led me to the solution:
Fatal error: Uncaught Error: Call to undefined function bccomp() in
/var/www/html/wp-content/plugins/civicrm/civicrm/CRM/Financial/BAO/Payment.php:374
..

this prompted me to install bcmath:
sudo apt-get update -y 
sudo apt-get install -y php7.2-bcmath 

restarted apache and the problem went away.