While what Dan said is true, the actual device should have no impact on your ability to log in to your Roku account via a Web browser.
Are you attempting to use a mobile device or a computer? On mobile devices, sometimes the default browsers don't work as well. You might want to ensure you are using either Firefox, Chrome, or Edge, all three of which are available on iOS and Android.
Regardless, on both mobile or computer, sometimes the browser will remember your password and fill it in for you. Sometimes, this could even overwrite what you typed in. If you have the ability to look at your browser settings for stored passwords, check to see if your browser is storing the password for Roku.com and, if so, delete that entry/entries.
Some password managers will also overwrite things you type in. LastPass is particularly guilty of this behavior. At least, it was, and that contributed to my switching to a different password manager. Disable any password manager plugins as you attempt to change your password.
Once that's done, if you have the ability to remove cookies from your browser, remove any and all cookies that contain "roku" in the domain. Also remove any related Local Storage, if that option is available.
Once that is done, try to log in to Roku on your browser (preferably by computer until this gets resolved). Write down what you think your password should be. Then write down a new password if you have to change it. Ensure it follows Roku's password rules.
Now, actually begin the process. First, go to
https://my.roku.com and attempt to log in to your Roku account with your current password. If it fails, don't try again. Simply click "Forgot password" and enter your email address on the page that displays. Don't forget to check the "I'm not a robot" box if presented. Submit it.
Next, open your email app. You should have an email from Roku with the subject "Roku | Forgot Password" that contains a password reset link. Click the link, and notice if it opens a tab in the same browser you were using, or if it opens a different browser.
If it opens a different browser (you were using Chrome and it opens Internet Explorer or Safari, for example), close that browser and go back to your email. Highlight and copy the link from the email. Paste it into a text editor and make sure there are no spaces in it. If not, paste the link into a new tab on the browser you were using.
You'll be prompted for a new password. Enter the new password you wrote down earlier. Make sure you write it exactly, as passwords are case sensitive. Once you've entered it in both fields, and ensured it knows you aren't a robot, submit the new password. If it accepts it, it will prompt you to log in.
Log in using the new password.
If anything doesn't respond as indicated or expected, stop. Let us know exactly where it broke down.
Basil
https://www.basilsblog.com/
Roku Ultra (4660)
Apple TV (5th gen), TiVo
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Roku boxes from every generation.
Apple TV (2nd, 3rd, 4th gen)