"Multiple primary keys for table are not allowed"

You marked two separate columns PRIMARY KEY on the same table. A table can only have one primary key — if you meant both columns together should be unique, that's a composite key, different syntax.

• SQLite: table "order_items" has more than one primary key
• PostgreSQL: ERROR: multiple primary keys for table "order_items" are not allowed
• MySQL: Multiple primary key defined
• SQL Server: Multiple primary keys cannot be created on table 'order_items'.

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The cause

PRIMARY KEY on a column isn't just "this column needs an index" — it's a declaration of the thing that identifies a row in this table. There can only be one. Writing PRIMARY KEY after two different columns looks like it means "both of these together uniquely identify a row," but that's not what the syntax says — it declares two separate, conflicting primary keys, which the engine refuses outright rather than guess which one you meant.

The fix — table-level composite key syntax

What was actually meant here is almost always a composite primary key — order_id and product_id together uniquely identify a row (one row per product per order), even though neither one does alone. That needs the table-level form, not two column-level PRIMARY KEYs:

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Now (1, 100) can only appear once, but 1 alone (as order_id) and 100 alone (as product_id) can each recur across different rows — one order can have many products, and one product can appear on many orders, just not the exact same pairing twice:

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That fails — (1, 100) already exists — which is the composite key doing exactly its job.

If you genuinely need two independent uniqueness rules

That's not a primary key question at all — a table can have one primary key and any number of separate UNIQUE constraints on other columns:

CREATE TABLE users (
  user_id INTEGER PRIMARY KEY,
  email   TEXT UNIQUE,     -- a second, independent uniqueness rule
  handle  TEXT UNIQUE       -- and a third
);