"My file won't upload" — usually the size: each file-upload question has a limit (your setting; default 10 MB, max 100 MB) and shows it. Very large videos are the classic case — raise the question's limit and publish changes, or ask for a smaller export. Also check your storage meter isn't full (see “Storage: what counts and what happens when full”). If you restricted file types, the form only accepts those (e.g. images and PDF).
"It says I already answered" — the form uses once per device or once per network (see “Response limits and one-answer-per-person”). Intentional? Then all is well. If the person legitimately needs another go (shared office network!), switch the policy to multiple temporarily, or have them use another network/device.
"My unlock code doesn't work" — codes are case-sensitive; typos with similar characters (O/0) are the usual culprit. Copy-paste the code from your settings and resend it. Deleted codes no longer work — check the code still exists.
"The form is in the wrong language" — the switcher sits at the top of the form (if you enabled translation). If a language is missing entirely, check your translation settings include it and re-publish (see “Automatic form translation”).
Tip
For anything else, ask for three things: the exact message, a screenshot, and device/browser. With those, support can usually answer in one reply.