I was getting this error for a different scenario. Here is my scenario.
(It’s very unlikely that anyone else ends up with this case, but incase)

I had a query working on a Table (say table A). Table A had a partition key m_id and sort key u_id.
I had a query to fetch data using m_id. The query was working.

”’

var queryParams = {
        ExpressionAttributeValues: {
          ':m_id': mId
         },
       KeyConditionExpression: 'm_id = :m_id',
       TableName: "A"
      };
    
   let connections = await docClient.query(queryParams).promise();

”’

I created another Table say Table B. I made some errors in naming keys so I simply deleted and created a table with the same name again, Table B. Table B had partition key m_id, and sort key s_id.

I copied pasted the same query which I was using for Table A, I changed Table name only because partition key had the same name.

To my shock, I get this expectation.

“ValidationException: Query condition missed key schema element”

I rechecked all the names, I compared the query with the working query. Everything was fine.
I thought maybe because, I was deleting recreating Table B, it could be something with that. So I create a fresh Table with a new Name Table B2 with the same key names as Table B.

In my query that was throwing exceptions, I changed only the Table name from B to B2.
And the Exception was gone.

If you are getting this on a fresh table, where no query has worked earlier, creating a new Table with a new name is an option.

If you delete a Table only to change partition key names, it may be safer to use a new name for Table as well (Dynamo could be referring metadata by table names and not by internal identifiers, it is possible that old metadata stays even if you delete a table. Just a guess given I faced this case).