Well,
In Business Central standard processes, when you "purchase" some work for an external vendor, you arent purchasing a person or 3 hours from a specific person, you are purchasing a service, and the external and services vendor gives you an invoice for their services. BC is expecting a purchase invoice for a GL account maybe called "external services" or maybe "external resources worked hours".
Some times, when I want to post the hours against the job line, i put gl acc with a suitable account, qty = 3 and unit price per hour in the purchase line and then link the job and job line, can i see the hours in the job line? yes, if you select the job line type in the purchase order


Remember, im talking about standard processes in BC
If you try to select Resource, you will need to create the external resource as internal in your resource table, an special resource posting group for them and set that posting group to an specific gl account because you are assigning a cost., take care of that.