I can open a file but cannot edit it

You have view or comment access. To ask for more, click your role next to the file name at the top of the document or spreadsheet and choose Request edit access

The owner gets an email and sees your request in their own Share dialog, where they approve or deny it. You get an email when access is granted

A file can also be read-only for a moment when many people edit at once. Up to 8 people can edit the same file at the same time