[Resolved] Solving theme conflict on Stripe buttons

I put Stripe pay now button on the WordPress theme I created, but both SCA and legacy pay now button won’t work correctly.
They work fine on the default WordPress theme such as Twenty Twenty, so I know this is a theme conflict, but I’m wondering if there is any way to make it work on my WordPress theme.

Enabling the SWPM debug mode does not show anything.

[SCA pay now button]
The first time I click on the Stripe SCA pay now button, it works fine.
After second time I click on the button, the page says:
Fatal error! Payment with ref_id swpm_(***payment ID?***)|(***button ID goes here***) can't be found

[Legacy button]
Every time I click on the legacy pay now button, the page says (with WP_DEBUG turned on):

Fatal error: Uncaught Stripe\Error\InvalidRequest: Must provide source or customer. in /wp-content/plugins/simple-membership/lib/stripe-gateway/lib/ApiRequestor.php:210 from API request 'req_wulQm1nsV3Moln' Stack trace:
#0 /wp-content/plugins/simple-membership/lib/stripe-gateway/lib/ApiRequestor.php(173): Stripe\ApiRequestor::_specificAPIError('{\n "error": {\n...', 400, Object(Stripe\Util\CaseInsensitiveArray), Array, Array)
#1 /wp-content/plugins/simple-membership/lib/stripe-gateway/lib/ApiRequestor.php(473): Stripe\ApiRequestor->handleErrorResponse('{\n "error": {\n...', 400, Object(Stripe\Util\CaseInsensitiveArray), Array)
#2 /wp-content/plugins/simple-membership/lib/stripe-gateway/lib/ApiRequestor.php(126): Stripe\ApiRequestor->_interpretResponse('{\n "error": {\n...', 400, Object(Stripe\Util\CaseInsensitiveArray))
#3 /wp-content/pl in /wp-content/plugins/simple-membership/lib/stripe-gateway/lib/ApiRequestor.php on line 210

[What I use]
– WordPress 5.4.2 (latest)
– SWPM 3.9.8 (latest)
– PHP 7.2

Thanks in advance,
eh2