I can't upload at all

If you never even get as far as picking files, that isn’t a bug in the app – it’s a permission or a state of the event. The upload dialog tells you with a padlock icon which case applies. If the upload does start and individual files fail, read My upload stops or fails instead.

The host turned guest uploads off

You see No Upload Permission with the text “You don’t have permission to upload content to this event. You can browse and view the gallery instead.”

The gallery is then deliberately view-only. The host can change that at any time – just ask. Where that is set is described in Roles & permissions.

Access tab in the event settings with the gallery protection section — On the host's side this is not a switch but a role: the **Access** tab holds the default role below gallery protection. **Viewer** means look and like only, **Contributor** means upload. With invitation codes the role sits in each code instead. ① the tab row with **Access** open, ② **gallery protection** with three choices: **Password Protected** (guests need a code), **Completely Private (Advanced)** (only invitations you create yourself work, the default code is off) and **Open Access** (anyone with the link can view), ③ **Save Changes** at the bottom. Further down the page come the guests' default permissions and the **Invitation codes** card.

Your role is viewer only

Whether you may upload depends on your role in the event:

Viewers can look and like, but never upload. No host setting changes that.
Contributors and moderators can upload as long as guest uploads are allowed for the event.
Admins and the owner can always upload while the event is running.

Only the host can change your role. More on this: Roles and permissions.

You have been banned from uploading

You see Upload Banned with the text “A moderator has restricted your ability to upload content. Please contact an event administrator for more information.”

This only affects new contributions. The photos and comments you already posted stay intact.
A ban can cover commenting as well.
Only a moderator or admin of the event can lift it, see Banning users.

Banned users tab with filters and user cards — This is the ban seen from the other side: the host manages it in the “Banned users” tab.

The event has ended or expired

Event Has Ended – “This event has ended and is no longer accepting new uploads. You can still browse and view the gallery.”
Event Has Expired – “This event has expired and is no longer accepting new uploads. You can still browse and view the gallery.”

In both cases uploads are closed for every guest, not just for you. Whether and for how long the gallery stays reachable afterwards is covered in How long do photos stay online?

The photo task is still locked

If you upload through a task, the task itself can be the reason:

The task card carries the chip Locked Task and the note “This task is locked. Complete the requirements to unlock it.” Under Requirements to unlock: you see what is still missing.
Some tasks are time-scheduled and only become available later – or are already over.
If a file is rejected for this reason, the button Skip Task appears next to Try Again. The photo then lands in the gallery as usual, just without the task assignment.

How tasks get unlocked is explained in Solving photo tasks.