In your Azure DevOps project navigate to Project Settings > Service Connections and select your Service Connection to Azure which will look something like this:

Depending on how your Service Principal was set up you may be able to browse to it directly in the Azure Portal by clicking the Manage Service Principal
link. There are other ways to set up a Service Connection and the method to find the Service Principal in the Azure Portal will differ, checkout the documentation here. Make a note of the Display Name for your Service Principal.
In the Azure Portal navigate to your AKS resource then navigate to the Access Control (IAM) blade and click Role Assignments. Check what Roles the Azure AD Service Principal for your Azure DevOps Service Connection has been assigned, it will need at least Contributor to make changes to your AKS cluster.