First can you please confirm the error is coming due to our plugin. You can do this by disabling the plugin and trying to checkout. If the error persists it is likely not from our plugin.
Usually, this error is due to lack of sufficient resources on your server. Could you possibly try increasing the memory limit?
Please let me know if the problem persists, in that case would it be possible for you to share your WordPress dashboard access and we can take a look for you? You can send your details to [email protected]