Troubleshooting common errors in Power Query involves identifying the root cause of the issue and applying the appropriate fixes. Here are some common errors and how to resolve them:

1. Data Source Errors
Issue: "DataSource.Error"

Solution:
- Check Data Source Connection: Ensure the data source is accessible, and the connection string or path is correct.
- Credentials: Verify that the credentials provided have the necessary permissions to access the data.
- Network Issues: Check for network connectivity issues or firewall settings that might block access.

2. Formula.Firewall Error
Issue: "Formula.Firewall: Query 'QueryName' (step 'StepName') references other queries or steps, so it may not directly access a data source. Please rebuild this data combination."

Solution:
- Privacy Levels: Adjust the privacy levels for the data sources. Go to `File` > `Options and settings` > `Options` > `Current Workbook` > `Privacy` and set it to `Ignore the Privacy Levels`.
- Combine Queries Carefully: Ensure that combining queries is done properly, avoiding direct references that violate privacy levels.

3. Data Type Errors
Issue: "DataFormat.Error" or "We couldn't convert to Number/Date/etc."

Solution:
- Check Data Types: Ensure the data types for columns are set correctly. Go to the `Transform` tab and set the appropriate data type for each column.
- Clean Data: Remove or replace non-conforming data. Use the `Replace Values` feature to handle unexpected data types.

4. Row-Level Errors
Issue: Rows containing errors due to data type mismatches or invalid operations.

Solution:
- Remove Rows with Errors: Go to the `Home` tab > `Remove Rows` > `Remove Rows with Errors`.
- Replace Errors: Select the column with errors, then go to the `Transform` tab > `Replace Errors` and specify a value to replace the errors.
- Conditional Handling: Use the `try ... otherwise` construct in M code to handle errors gracefully:
```m
Table.TransformColumns(PreviousStep, {{"ColumnName", each try _ otherwise "Default Value"}})
```

5. Query Folding Issues
Issue: "The query contains transformations that cannot be folded back to the data source."

Solution:
- Minimize Complex Transformations: Simplify transformations that can be performed directly on the source.
- Check Query Folding: Right-click a step in the Applied Steps pane and select `View Native Query` to see if query folding is occurring. If not, consider performing complex transformations later in the query.

6. Memory or Performance Issues
Issue: Power Query runs slowly or fails due to memory constraints.

Solution:
- Optimize Queries: Reduce the number of steps in your queries and avoid unnecessary transformations.
- Filter Early: Apply filters early in the query process to reduce the amount of data being processed.
- Increase Memory Allocation: If possible, increase the memory allocated to Power Query by adjusting system settings.

7. M Code Syntax Errors
Issue: "Expression.Error: The name 'X' wasn't recognized."

Solution:
- Check Syntax: Review the M code for syntax errors, missing commas, or incorrect function names.
- Use IntelliSense: Use the IntelliSense feature in the Advanced Editor to help with code completion and error detection.
- Debug Step-by-Step: Add steps incrementally and check each one to identify where the error occurs.

8. Missing or Renamed Columns
Issue: "An error occurred in the 'StepName' step. The column 'ColumnName' of the table wasn't found."

Solution:
- Check Column Names: Ensure that column names have not changed in the source data. If they have, update the references in the query.
- Use Dynamic Column Names: Use dynamic column references where possible to avoid issues when columns are renamed.

By systematically addressing these common errors, you can effectively troubleshoot and resolve issues in Power Query.