A new OS comes with a lot of new features as well as some major problems. Same goes for Nokia 8. Still, smartphones are facing issues due to Oreo waiting for getting updates. One issue Nokia 8 users are facing after Oreo Update is SD card access failed.
Mostly SD card works fine and you can write files easily through any App to SD card. Though if you want to retrieve data from SD card using the same App if gives SD card access failed. For example you can capture pictures from your camera they would be saved to your card. But when you will try to open last captured image it will give an error.
How to Solve Nokia 8 SD card access failed
There is not a permanent fix for the solution. Hopefully phones will get an update soon which will fix it. Although you can get rid of file access by using your file explorer.
To open file explorer Go to Settings -> Storage -> Internal Storage -> Files.
Or simply open in using the files icon in the menu as shown below.
This will not fix your issue but will provide you an alternative to access your SD card storage.