Answer:
In English:
Thank you so much for the invitation but I’m afraid I’m not able to attend because I'm not feeling very well.
In Spanish:
Muchas gracias por la invitación, pero me temo que no podré asistir porque no me siento muy bien.
Step-by-step explanation:
This expression work equally well in spoken or written English so you could use them on the phone or in an email.