BCMath is a set of PHP functions that allow you to use Arbitrary-precision arithmetic. Am getting an error "Commerce requires the bcmath PHP extension." before a couple of months. To resolve this error we need to enable BCMath extension. following steps help you to resolve bcmath php extension error.
 

Verify PHP version.
Before going to the next step you need to check the PHP version. you can check PHP version with the following command. Before installing or enable BCMath you need to know PHP version.

Also Read - How to install PHP 7.2 on Centos 7

# php -v

Output:-

PHP 7.2.14 (built: Jan  8 2019 14:05:15) ( NTS )

Copyright (c) 1997-2018 The PHP Group

Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies

with Zend OPcache v7.2.14, Copyright (c) 1999-2018, by Zend Technologies

Install BCMath For PHP 7.0

- For centos or fedora

# sudo yum install -y php7.0-bcmath

-For Ubuntu users

# sudo yum install -y php7.0-bcmath

Install BCMath For PHP 5.5

For Centos or Fedora

# sudo yum install -y php5.5-bcmath

For Ubuntu users

$ sudo apt-get install php5.5-bcmath

Restart Apache server

After the PHP BCMath installation, we need to restart Apache service. With the following command.

# sudo service httpd restart