If you're using Google Data Studio for your Google Analytics reporting, it's likely you've logged in today and seen a system error for any widgets containing the 'Unique Event' metric:
Google Analytics have renamed the 'Unique Events' metric to 'Unique Dimension Combinations'. To add to confusion, they've deprecated the api field and also created a new metric called 'Unique Events' which is calculated differently to the previous Unique Events metric. For more details check out this post https://www.clickinsight.ca/about/blog/unique-dimension-combinations-udc
How To Fix This Issue
To resolve this issue and get your reports working again you'll need to refresh your report data source and update the metric in your widget.
Step 1 - Refresh Your Data Source
Firstly, in edit mode, click on the widget showing the system error and click on the pencil icon next to your data source:
Then click 'Refresh Fields':
Step 2 - Update Your Metric
Once the fields have been refreshed you will notice that the widget error and metric have updated as below:
At this point you will need to click on 'Invalid Metric' and select 'Unique Dimension Combinations' or 'Unique Events' depending on which metric you want to use:
Once you have selected your new metric your report widget should be working again:
You will need to carry out the above steps for all widgets showing this error.