Please ensure that you follow this for the plan creation:

resource flexFuncPlan 'Microsoft.Web/serverfarms@2024-04-01' = {
  name: planName
  location: location
  tags: tags
  kind: 'functionapp'
  sku: {
    tier: 'FlexConsumption'
    name: 'FC1'
  }
  properties: {
    reserved: true
  }
}

Here's a full example: https://github.com/Azure-Samples/azure-functions-flex-consumption-samples/blob/main/IaC/bicep/core/host/function.bicep