When the project is to import large blocks of data and then link them by internally generated primary keys such as your GuildID number, the trick is to create a second relationship that matches by some other value, then the Taxonomy::fk_GuildID can use this second relationship to lookup a value from GuildCodes::pk_GuildID so that you have the values in place to make the GuildCodes::pk_GuildID = Taxonomy::fk_GuildID relationship functional.
In Manage | Database | relationships, make a new table occurrence of GuildCodes by clicking it and then clicking the duplicate button (2 green plus signs). You can double click the new occurrence box to get a dialog to appear where you can rename the new occurrence box GuildCodesByName.
We have not duplicated a table. Instead, this is a new reference to the same table already present in your database.
Add it to your relationships like this:
GuildCodesByName::TaxonomicName = Taxonomy::TaxonomicName
You can now pull the ID code from GuildCodes across this relationship into fk_GuildID in one of two ways:
1) Set up Taxonomy::fk_GuildID with a looked up value or auto-entered calculation field option to copy GuildCodesByName::pk_GuildID into the field.
2) Immediately after import, use Replace field contents to popuplate the GuildCodes::fk_GuildID with values from GuildCodesByName::pk_GuildID.