Cannot modify header information

I have this plugin installed and its working great at https://wyomingbackpacks.org/ but as Im building the site, everytime I install and activate a plugin, I get messages about ‘cannot modify header information’ . See this screenshot when I activated ‘all in one seo’ https://take.ms/uz4Ma. I get this for any plugin. I can hit the back button and see that the plugin DID activate and it works fine.

can you please help?

https://www.ads-software.com/extend/plugins/ultimate-coming-soon-page/