This most commonly occurs on Android devices.
Try the following:
Go to Settings > App & Notifications > select Salesforce1 > Advanced Settings > Open by default > Open supported links > and change to "Ask everytime".
Then restart Scan to Salesforce and try scanning again.
When you’re asked to select Chrome or the Salesforce app, choose Chrome and log in to Salesforce in that browser.
*Make sure you’ve installed the latest version of the mobile app.
*See the Administrator Guide regarding login errors with other devices.