warning on admin login

I am having an issue logging into my site, and when I try to go to the admin page, I am getting this message:

Warning: Cannot modify header information – headers already sent by (output started at folders\wp-content\themes\canvas\functions.php:1) in functions\wp-includes\pluggable.php on line 881

I have seen on some forums that it can be the spave before or after the coding in the .php files, and I have tried to do this, but it is still not working.

I have put the coding of this pluggable file into pastebin:

https://pastebin.com/M67LAALh

the coding on the 881 line that is apparently causing the problem is:

header(“Location: $location”, true, $status);

Does anyone have any ideas? This is driving me crazy!