The error "Inventory account does not exist" means that there is no account which has the value "Inventory" for the drop-down "Summary account for". (This leads to the value "IC" in the column "Links" of the chart of accounts overview under the menu General Journal > Chart of accounts -- see https://book.ledgersmb.org/1.3/full-book/images/setup-bank-account1.png
Referring to the same images: The error "Income account does not exist" means that there is no account which has a checkmark "Income" on the line "Tracking items" for parts or on the line "Non-tracking items" for services. (These checkmarks can be identified in the Chart of accounts overview as "IC_income" and "IC_sale" respectively.)
Did you start with a pre-defined chart of accounts which comes with the software or are you setting up your own? If it's one of the pre-defined ones, please let me know which one it is so we can fix it.
Hope that helps!