Secret Manager access issues - google-cloud-platform

Question

I'm trying to incorporate Secret Manager with my projects for security but running into issues setting it up. I currently have a service account in project-b where I downloaded the JSON credential keys and have been using that to access my BigQuery table in my backend code.
My current setup:
I have project-a that uses Cloud Run to host my code.
I have project-b that uses BigQuery to hold some data for me.
From project-a, I'm trying to access the BigQuery table in project-b just like I've been doing with the JSON keys.
I keep running into this error:
PermissionDenied: 403 Permission 'secretmanager.versions.access' denied for resource 'projects/project-b/secrets/stockdata-secret/versions/1' (or it may not exist).
I have assigned the Secret Manager Secret Accessor and Secret Manager Viewer roles to a couple of my accounts but it still doesn't seem to work.
The client_email from the keys is set to the top service account in the screenshot below:
Permissions for the secret
Here is my part of my back-end code:
# Grabbing keys from Secret Manager, got this code from Google docs
def access_secret_version(project_id, secret_id, version_id):
# Create the Secret Manager client.
client = secretmanager.SecretManagerServiceClient()
# Build the resource name of the secret version.
name = f"projects/{project_id}/secrets/{secret_id}/versions/{version_id}"
# Access the secret version.
response = client.access_secret_version(request={"name": name})
payload = response.payload.data.decode("UTF-8")
return payload
---
# Routing to the page
#app.route('/projects/random-page')
def random_page():
payload = access_secret_version("project-b", "stockdata-secret", "1")
# Authenticating service account.
os.environ["GOOGLE_APPLICATION_CREDENTIALS"] = payload
# old way, which worked
google_cloud_service_account = "creds.json"
os.environ["GOOGLE_APPLICATION_CREDENTIALS"] = google_cloud_service_account