What is causing Foreign Key Mismatch error?

I'm not familiar with SQLite but a little Google'ing turned up this. The documentation says

If the database schema contains foreign key errors that require looking at more than one table definition to identify, then those errors are not detected when the tables are created. Instead, such errors prevent the application from preparing SQL statements that modify the content of the child or parent tables in ways that use the foreign keys. Errors reported when content is changed are "DML errors" and errors reported when the schema is changed are "DDL errors". So, in other words, misconfigured foreign key constraints that require looking at both the child and parent are DML errors. The English language error message for foreign key DML errors is usually "foreign key mismatch" but can also be "no such table" if the parent table does not exist. Foreign key DML errors are may be reported if:

• The parent table does not exist, or
• The parent key columns named in the foreign key constraint do not exist, or
• The parent key columns named in the foreign key constraint are not the primary key of the parent table and are not subject to a unique constraint using collating sequence specified in the CREATE TABLE, or
• The child table references the primary key of the parent without specifying the primary key columns and the number of primary key columns in the parent do not match the number of child key columns.

I suspect you might be running into #3 in that list.

Also, while other DBs might support using a non-unique index as a foreign key reference, (see answers here), it's a bad design choice in my opinion. I would restructure so that either

1. Reading.PatientId references Event.PatientId so that the complete composite key from Event is referenced by Reading or,
2. Add an EventId auto-increment, primary key to the Event table and use that as the foreign key in the Reading table (so that you only have EventId and Value under Reading and you can get the PatientId, DateTime, EventTypeCode out of Event).

I'd suggest #2 so that you can avoid the redundancy of PatientId, DateTime and EventTypeCode in both Event and Reading.