Question

How would you decline an invitation because you don’t feel well?

Answer 1

Lo siento, no puedo ir. No me siento mejor.

Answer 2

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.