Installation Failed / Deletion Failed on any plugin

I have a simple wordpress site that can’t install or remove a plugin without stating items like the following:

Deletion failed: {“success”:true,”data”:{“delete”:”plugin”,”slug”:”hello-dolly”,”plugin”:”hello-dolly\/hello.php”,”pluginName”:”Hello Dolly”}}

I get similar errors if I install a plugin (installation failed)

Then when I try to activate a plugin, I get

Warning: Cannot modify header information – headers already sent by (output started at /home/domain.com/domain.com/wp-content/themes/AdventureWP/functions.php:5) in /home/domain.com/domain.com/wp-includes/pluggable.php on line 1174

Warning: Cannot modify header information – headers already sent by (output started at /home/domain.com/domain.com/wp-content/themes/AdventureWP/functions.php:5) in /home/domain.com/domain.com/wp-includes/pluggable.php on line 1174

Where do I look for the issue? It’s a pretty fresh install of wordpress