Square payment errors - internal server error

I have installed https://www.opencart.com/index.php?rout ... n_id=31311 this extension on a clients opencart 2.3.0.2 shop website and after putting in test card details I got from Square website, it comes up with Internal server error when clicked on the confirm order button

The errors logs from the FTP server show the following

[22-Feb-2021 16:25:31 UTC] PHP Fatal error: Uncaught Error: Call to undefined function vendor\isenselabs\Squareup\bcmul() in /public_html/shop/system/library/vendor/isenselabs/squareup/api.php:789
Stack trace:
#0 /public_html/shop/catalog/controller/extension/payment/squareup.php(206): vendor\isenselabs\Squareup\Api->lowestDenomination(909.05, 'GBP')
#1 /public_html/shop/vqmod/vqcache/vq2-system_storage_modification_system_engine_action.php(51): ControllerExtensionPaymentSquareup->checkout()
#2 /public_html/shop/catalog/controller/startup/router.php(25): Action->execute(Object(Registry))
#3 /public_html/shop/vqmod/vqcache/vq2-system_storage_modification_system_engine_action.php(51): ControllerStartupRouter->index()
#4 /public_html/shop/system/engine/front.php(34): Action->execute(Object(Registry))
#5 /public_html/shop/system/engine/front.php(29): Front->execute(Object(Action))
#6 /public in /home/morleycomputers/public_html/shop/system/library/vendor/isenselabs/squareup/api.php on line 789

Has anyone had these issues before or know how to fix it, I did think if is related to Square rolling out the SCA compliant flow with 3D-Secure 2 (3DS2)

Caused by an installed extension. Contact iSenseLabs to resolve this issue.

Thank you, I have just opened a support ticket with them