At the very least you will need to set a default circulation rule. Koha does not come with one by default. This rule should be set for the default itemtype, default branchcode and default patron category. That will catch all instances that do not match a specific rule. When checking out if you do not have a rule for the default branch, default item and default patron category then you may see patrons getting blocked from placing holds.
Also check that the AllowOnShelfHolds system preference is turned on – it is off by default in a standard Koha. Turning this system preference on allows items to be placed on hold even if the item is currently checked-in.
Posted in: Circulation