The Dronelink app (web/native) and the support center (forums, etc) use different login systems. Depending on which system you are attempting to access, you need to create an account for that system as there is no single-sign-on functionality at this time.
If you already have an account for the native app and cannot login, make sure you have verified the email address associated with your user profile. You may need to check your spam folder to find the verification email.
Additionally, for the support center / forums, you need to enable cookies and cross-site tracking in your browser to login to the support center. For example, if you are using Safari, these are the settings to use in the Privacy tab (you may need to restart Safari after changing the settings):