My entry will not save
A refused save is almost always one of five things: a required field left empty, a value outside a field's allowed set, a duplicate in a field that must be unique, a link pointing at something that does not exist, or — for bookings — a slot with no capacity. The message names the field; fix that field rather than retrying.
You fill in a record, save, and it comes back refused. The refusal is deliberate — the entry is checked against the rules of its collection before anything is stored, and the message names the field that failed. Start there rather than retrying the same save.
1. A required field is empty
Some fields are marked required, meaning an entry is meaningless without them. Fix: fill it in. If a field is required but you genuinely do not have the value at the time of entry, that is a signal the field should not be required — change it on the collection rather than inventing placeholder values.
2. A value is outside what the field allows
A field with a fixed set of options accepts only those options, and a typed field accepts only that type — a number field will not take text, a date field will not take a loose phrase. Fix: choose from the allowed values, or correct the format. If the value you need genuinely is not among the options, add it to the field's options rather than forcing it elsewhere.
3. A unique field already has that value
A field marked unique cannot repeat across entries — an order number, a code, an email that must identify one record. Fix: search the collection for the existing entry. Nine times out of ten the record already exists and you are creating a duplicate; update the existing one instead.
4. A link points at something that does not exist
A reference field must point at a real entry in the collection it targets — you pick an existing record, you do not type a name. Fix: create the target entry first, then link to it. "The customer does not exist yet" is the usual story: add the customer, then the order.
5. The booking slot has no capacity
If the collection takes bookings, a save can be refused because the time asked for is not available — outside the department's working hours, or already full. Fix: check the department's hours and existing bookings for that slot. If nothing at all is bookable, the working hours are probably not set — see set up locations, departments, and working hours.
Still refused?
Two less obvious causes. A computed field cannot be written to — rollups and lookups are calculated, so filling one in by hand or through an import is refused by design; leave them alone and let them update. And a conditional rule on the collection may make one field's allowed values depend on another — in which case fix the field it depends on first, and the blocked field will accept what you need.
Related
Field types and their settings are covered in create a collection; the difference between linking and computing in reference vs multi-reference and rollup vs lookup.