Access error message: The key was not found in any data record

1. when does this error occur?

For imports from CSV or Excel, for example:

If you have a table created / linked from an import and specify that the first row contains the headings.

Why does this error occur?

Quite simply, in the import there are, for example, column headings that are repeated or the column headings contain names that are invalid for Access.

In databases, a column / data field must be unique for a data record.

Solution:

Edit the data source to be imported and ensure that all column headings are unique.

2. when does this error occur?

For queries:

This message is very tricky. It may be correct because no data record actually matched the search criteria, but it doesn't have to.

Why does this error occur?

Under Office 2007 and 2010, I was able to observe that this message was an indication of a structural defect within a table.

Queries are also about ensuring that all data fields are unique and can be assigned to a table.

Solution:

Before a Create a backup copy of the database, then compress and repair the database.

If this did not help, then delete individual fields one by one from the query or its subqueries until the culprit is found.