This is most likely if you're using an Android device.
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 it asks to select Chrome or the Salesforce app, select Chrome and log in to Salesforce on that browser.
*Please see Administrator Guide for login error with other devices.