First login, and you don't have a password yet? On https://app.volta-back.com/login or in the app, enter your email address, then "Forgot password?" (website) or "Reset password" (app) to set one: see "How do I reset my password?". The "Activate my account →" link in the invitation email does the same thing: it is valid for 14 days and can only be used once. Only the latest email you received works.
No email, or the message "Invalid email or password."
If no email arrives, your account may not have been created: check with your fleet manager (see "I didn't receive my Voltaback invitation email"). An unknown address shows the same message as a wrong password: check your address. In the app, the message is "Incorrect email or password. Please try again.".
Mobile app
It is reserved for drivers. A fleet manager account sees "This account is not authorized for the mobile app. Only employee accounts can sign in.": sign in on the website.
Single sign-on (SSO)
If your company uses it, the app shows "Continue with SSO"; on the website, you are redirected to the "Sign in with SSO" page: see "How do I enable automatic sign-in (SSO)?".