No General Posting Setup for this group combination
Business Central decides which G/L accounts to use from two codes: the Gen. Bus. Posting Group taken from the customer or vendor, and the Gen. Prod. Posting Group taken from the item, resource or G/L account. The message says no line exists in General Posting Setup for that exact pair, so there is no account to post to. Create the missing line for the two values shown in the message, or correct the posting group on the record that carries the wrong code.
General Posting Setup is a matrix. Every pair of business group and product group that occurs in real documents needs its own line, and the line carries the G/L accounts for sales, purchases, credit memos, discounts and inventory adjustments. Implementations set up the pairs that were in use on day one, so the error appears later, when a new code is introduced on one side and combines with codes on the other side for the first time. Blank counts as a value here, which is why transfers, assembly, production and some service postings hit combinations that nobody set up.
Keep the number of posting groups as small as the accounting really requires, and treat the matrix as complete: whenever a new business or product group is created, add every line that the new code can produce, including the combinations with a blank group. Review the table after any data migration or company copy, and test one sale, one purchase and one credit memo in a sandbox before the new group goes live. The accounts belong to finance, so agree them with the person who owns the chart of accounts rather than deciding them at posting time.
No. The account decides where the amount lands in your financial statements, and a wrong entry has to be reversed later. Copy the accounts from a comparable line and have finance confirm them.
Because blank is a valid posting group. Transfers, assembly and some production or service postings run with an empty business group, so the setup needs a line with that field left empty.