try this way of substituting in branch

bitbucket-pipelines.yml

@@ -139,6 +139,14 @@ pipelines:
       - step:
           name: Deploy K8s apps (staging)
           script:
+            # Set a default branch for local testing or safety if BITBUCKET_BRANCH is not always present
+            - export BITBUCKET_BRANCH=${BITBUCKET_BRANCH:-notarealbranch}
+
+            # Template argocd/base/applications.yaml in place for the current pipeline run
+            - envsubst < k8s/argocd/base/applications.yaml > k8s/argocd/base/applications.yaml.tmp
+            - mv k8s/argocd/base/applications.yaml.tmp k8s/argocd/base/applications.yaml
+
+            # Now run kubectl apply -k. Kustomize will pick up the modified base file.
             - pipe: atlassian/kubectl-run:3.1.2
               variables:
                 KUBE_CONFIG: $KUBE_CONFIG_STG

k8s/applications/templates/argocd.yaml

@@ -10,7 +10,7 @@ spec:
   source:
     path: k8s/argocd/overlays/{{ .Values.environment }}
     repoURL: 'git@bitbucket.org:jamkazam/video-iac.git'
-    targetRevision: {{ .Values.gitBranch }}
+    targetRevision: ${CI_BUILD_BRANCH}
   project: default
   # syncPolicy:
   #   automated: