Your SPF records should be on the root domain, not the www subdomain. So set the TXT record on website.com instead of www.website.com and that should give you what you need.

From your screenshots it looks like you're trying to set up the www subdomain in Mailgun, which probably isn't what you want. Change to just the root domain there, otherwise you may be sending something@www.website.com emails.

Your SPF records should be on the root domain, not the www subdomain. So set the TXT record on example.com instead of www.example.com and that should give you what you need.

From your screenshots it looks like you're trying to set up the www subdomain in Mailgun, which probably isn't what you want. Change to just the root domain there, otherwise you may be sending something@www.example.com emails.