If this screenshot is not enough to guide you, read the text below it.
If you need to access a file using a link to it that you got by email, you must be logged in to Google with the correct account in order to access it.
If you're reading this article, it's probably because you clicked on the link and got a notice from Google Drive that You need permission, and you didn't read the notice (ahem) and clicked on the blue "Request access" button, and then I told you that you need to switch accounts and directed you here.
So stop and think which Google Account you must be logged in to in order to get the permissions. If I told you, you know; but if that's not how you got here, it's a virtual certainty that the account is the one where you're reading the email with the link.
Now, you would think that if you're logged in to an email account, the system would automatically know to use that account. But that's not how browsers and/or Google apps work. Sorry, fixing that is above my pay grade. Sometimes you just need to switch to the correct account.
The screen that you see is probably similar to the one at the top of this article. Look at the boxes I outlined in red. If you're still unsure what to do, ask me, but make sure to tell me that this article didn't fix your problem, because otherwise I'll just send you back here.