adding new column in dynamo DB using boto3 resource with update_item

I need to update a row in Dynomodb while doing so need to include a new column which is already not there.

resp = table.update_item(
            Key={
                'Pkey': 'key1',
                'Skey': 'skwy2'
            },
            UpdateExpression='ADD dateModified = :input2, SET IsActive = :input1',
            ExpressionAttributeValues={
                ':input1': False,
                ':input2' :  datetime.datetime.now(timezone.utc)
            },
            ReturnValues="UPDATED_NEW"
        )

now I need to update IsActive field to false and insert new dateModified value to that .

getting error as Invalid UpdateExpression: Syntax error; token: "=", near: "dateModified = :input2"