Conditional formatting on grouped reports
Key Takeaway
Conditional formatting works only on grouped reports (summary or matrix) that contain at least one summary field or custom summary formula.
Conditional formatting colors the summary values in a grouped report, using number ranges and colors you choose. For example, low totals can show in one color and high totals in another. Readers can then spot high, middle and low results at a glance without reading every number.
Which reports can use conditional formatting
Conditional formatting works only on grouped reports: a summary report (grouped by rows) or a matrix report (grouped by rows and columns). In a matrix report, you can also apply conditional formatting to grand totals.
The report must also contain at least one summary field or custom summary formula, because each rule is built on one of those fields.
If a report isn't grouped...
You need to group it by rows or by rows and columns first.
If a report has no summary field or custom summary formula...
You need to add one before you can add a rule.
Joined reports can't use conditional highlighting. Conditional formatting also isn't yet available for joined reports in Lightning Experience. If a requirement calls for color-coded ranges, use a summary or matrix report instead of a joined report.