To add an SPF record, you will need to manage your TXT records. The default SPF record for Hostinger emails will be:
v=spf1 include:_spf.mail.hostinger.com ~all
If your domain is pointing to Hostinger by nameservers, you can set correct email DNS records from hPanel: just go to Emails → domain name → DNS settings and click on Set all to default:
Once the SPF record is successfully added propagation will start, which can last up to 24 hours to fully propagate.