How to resolve "Document not relevant for billing" error message?

Check the detailed error log in VF01 screen. We may get more information on error.

Then, check have you completed the PGI for the delivery document or not.

Next, go to that Item Category functionality VOV7 and find if it is order or delivery related billing.

And after that go and check in the particular Sales order document type whether billing document types are maintained in the sales document or not.

If every thing is correct go and find with copy controls

If it order related, go VTFA
If it Delivery related, go VTFL and check in your document type combination is maintained or not, if they are maintained item category is available or not.

After delivery while creating billing document system showing error that no billing document is generated for material no 395.  So how I can solve this issue.

Check if Item Category is checked for Billing or not.

Normal Sales Order Cycle:-

Step 1: Sales Document Type
IMG > Sales and Distribution > Sales > Sales Documents >
Sales Document Header:
1. Sales Document Type:The sales document types represent the different business transactions, such as Inquiry, Quotation, Sales Order, etc. To create new sales order type, always copy as with reference to similar sales order. If possible use standard sales order.
2. Define Number Ranges For Sales Documents: Maintain number range with discussion with core team.
3. Assign Sales Area To Sales Document Types: 
A. Combine sales organizations / Combine distribution channels / Combine divisions: Ensure to maintain these, else Sales Order creation will give error.
B. Assign sales order types permitted for sales areas: Assign only required Sales Order Types to required Sales Area. This will minimize selection of Sales Order Type as per sales area.

Sales Document Item:
*1. Define Item Categories: If possible use Standard Item Category. Incase if required to create new, copy as from standard & maintain New.
*2. Assign Item Categories: If possible, use standard. Formula for deriving item category: Sales Document Type + Item Category Group + Usage + Higher Level Item Category = Item Category

Schedule Line:
1. Define Schedule Line Categories: If possible use Standard Schedule Lines. Incase if required to create new, copy as from standard & maintain New.
2. Assign Schedule Line Categories: If possible, use standard. Formula for deriving Schedule Line: Item Category + MRP Type / No MRP Type.

Step 2:
IMG > Logistic Execution > Shipping > Deliveries > 
*1. Define Delivery Types: If possible use Standard Delivery Type. Incase if required to create new, copy as from standard & maintain New.
*2. Define Item Categories for Deliveries: If possible use Standard Item Categories for Delivery Type. Incase if required to create new, copy as from standard & maintain New.
*3. Define Number Ranges for Deliveries: Ensure to maintain number range.

Step 3: 
IMG > Sales and Distribution > Billing >
1. Define Billing Types: If possible use Standard Billing Type. Incase if required to create new, copy as from standard & maintain New.
2. Define Number Range For Billing Documents: Ensure to maintain number range.
3. Maintain Copying Control For Billing Documents: Maintain relevant copy controls such as Sales Order to Billing, Deliver to Billing, etc.

Note: Ensure that Copy Control settings are done
Sales Order to Sales Order (QT --> OR): VTAA
Sales Order to Delivery (OR --> LF): VTLA
Sales Order to Billing (OR --> F1): VTFA
Delivery to Billing(LF --> F2): VTFL
Billing to Sales Order (F2 --> RE): VTAF

The configuration differs from scenario to scenario & requirement of the client.

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