Error: Error ValidationException: One or more parameter values were invalid: Type mismatch for key X expected: S actual: M

When trying to put or update an item to DynamoDB, you might have tried to write it with incorrect attribute type for example number instead of string or the other way around.

Even though DynamoDB is schemaless, at the table creation you had an ability to specify some AttributeDefinitions. They need to be preserved for all the items in the table.