Create a filter on date field and select only July 2020 and Jan 2021.
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Drop Date Month Discrete to Columns and New cases to the Text.
Right-Click on New Cases field and Quick Table Calc ->Percent Difference Graphical user interface, application Description automatically generated

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Explanation
As per the Tableau order of operation, dimension filter is applied before table calculation is computed. However, fixed LOD is executed before the dimension filter. That's why total sales are showing different results.
Reference -https://help.tableau.com/current/pro/desktop/en-us/order_of_operations.htm

Even if you don't select the target filters, your filter action will work. Rest all you need to provide so that it will work.
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Explanation
Refer:https://help.tableau.com/current/pro/desktop/en-us/dashboards_dsd_create.htm Go to the section- Phone layouts and the Default dashboard

Explanation
Drop the year to the Columns and change it Discrete. Drop the matches played to the Rows.
Go to the Analytics menu, drop the Distribution band to the view. Select scope as entire table.
Value Computation should be:

Your view will look like this: