To connect to your OneDrive for Business account, simply tap +Add > OneDrive and sign in with your account credentials. On iPhone, this option is available in Services > Add Account.
Note: in case conditional access is used for your account, the admin of your organization should configure the rights for your account as per the following article: https://docs.microsoft.com/en-us/azure/active-directory/active-directory-conditional-access-technical-reference
If you are not able to connect your OneDrive for Business account, drop Support team a line and we will get back to you as soon as possible. To troubleshoot the issue, please provide us with the following information: