The error indicates that the cost centre used on the detail line of the purchase request does not exist against the relevant project in Dimensions.
To add the relevant cost centre to the Dimensions project:
In Dimensions, click Costing, Projects
From the Project list, select the project used in the purchase request
Click Cost Centres (Cost Cnt)
Check the returned list of cost centres. You should find that the cost centre mentioned in the FocalPoint error is not listed.
Click Edit and add the relevant cost centre. Once added, your purchase request should then post as expected.