Why does GA4 say incompatible dimensions and metrics?

GA4 fields have a scope (event, session, user, item, cohort). Combining fields whose scopes can't be resolved together — for example an item-scoped dimension with a session metric, or a cohort field outside a cohort report — returns 'incompatible dimensions and metrics'. Remove the offending field, match scopes, or check each field's compatibility list.

“Incompatible dimensions and metrics”, explained

You added a field to an Exploration or a report and the numbers vanished, replaced by incompatible dimensions and metrics. It isn't a bug — it's GA4's scope model refusing an impossible question.

The rule

Every GA4 field carries a scope: event, session, user, item, or cohort. Some scopes can't be combined in one query because there's no single row grain that satisfies both. The classic triggers:

Item-scoped with non-item fields. Ecommerce item dimensions (itemName, itemBrand) only combine with item metrics.
Cohort fields outside a cohort report. cohort, cohortNthDay need a cohortSpec and only pair with cohort metrics.
Certain session/user mixes. A few user-scoped metrics won't resolve against session-scoped dimensions.

The fix, in order

Remove the last field you added — that's almost always the culprit.
Match scopes: swap a session dimension for its user equivalent (or vice versa).
Open the field's page here and read its compatibility list before adding it back.
In the API, call checkCompatibility to see exactly what remains valid with your current selection.

The deeper version of this problem isn't the error — it's the silent one.

Two fields that are technically compatible can still produce a wrong number if the reporting model joins them at the wrong grain — sessions inflating, one page split across thousands of URL rows. That's the class of bug Grain exists to fix. If your GA4 totals and your Power BI totals disagree, the free 12-question check is a good place to start.