Known Plugin Conflicts
When database upgrades are required by Payment Page, there is a conflict with the W3 Total Cache plugin because W3TC caches the WordPress REST API, which is required for database upgrades.
To solve this, clear the W3TC cache, temporarily disable W3TC during the migration upgrade process, or altogether use a different caching plugin.
Ensure that there are no performance or security plugins disabling the WordPress REST API. You should also verify with your hosting company they are not disabling the REST API, as it is necessary for payments.
We've had reports from some users that plugins like Perfmatters, which may disable the REST API, can block payments from completing (particularly on mobile). In the perfmatters plugin specifically, leave the settings like this:
Perfmatters Disable REST API Setting