Refresh your browser. Press CTR+SHIFT+R and try again.
Also to access the form, on Return dashboard, please do NOT click on “File GSTR-3B” button. If you do, you will get access denied or invalid return message. Please Click on “Monthly Return” written in Blue Box.
Even if after these steps problem is not solved, try another browser.
At last can say it may be due to GST server errors.