Shopify users must login via the Shopify backend.
Shopify users do not have an email and password based login, the only method of access is via the Shopify dashboard.
Open the Shopify dashboard
Click on 'Apps'
Click on the Promolayer app.
How to log in (Shopify users)
Step-by-step guide for Shopify merchants to log in to Promolayer and access their dashboard through the Shopify app.