When importing a configuration, it halts with this error message:
This message means that:
The fix to this problem depends really on what you want to do with the new configuration.
The straightforward fix would be to add config scheme YYYY to custom field XXXX in the origin so that the new configuration can be imported without needing to remove that config scheme. Then, export the configuration and try the load again. Configuring a custom field from Jira docs explains how to add a new config scheme (or custom field context) to a custom field. You need not worry about the options contained by this new config scheme. If the options used in the target instance do not exist in the new config scheme, the app disables them but does not remove them, so that existing data is preserved.
Other options that would minimize the occurrence of this problem: