If you don't see the Create Campaign button in your native Apple Ads account, the App Store Connect account has not been linked to your Apple Ads account.
Note: App Store Connect was previously called iTunes Connect.
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Connect your App Store Connect account to your Apple Ads account to see the Create Campaign button.
Here's a link to App Store Connect.
You can create a connection when creating an account or new campaign groups:
Sign in to your Apple Ads Advanced account and go to the Campaigns dashboard.
Click the drop-down menu in the upper-left corner next to the account's name.
Click Settings.
Link your Apple Ads Advanced account to your App Store Connect account.
After the two accounts have been linked, clear your web browser's cache and sign in.
Note: the App Store Connect Legal Account Holder, Admin, or Marketer should have the Apple Ads Account Admin or Campaign Group Manager role to link the App Store Connect account to the campaign group.
Here's what your account should look like in Apple Ads.
Since SplitMetrics Acquire pulls data from Apple Ads, you first need to fix this in your Apple Ads account, and it will be automatically set in your SplitMetrics Acquire account.