#robshit

(obviously I don't know the specifics of your work place and their policies so edit where needed to include any other necessary information.)

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Hi [manager's name],

I'm afraid I'm not going to be able to come in for my [insert start time] shift due to [illness/symptoms ect].

Sorry for any inconvenience.

[your name]

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I've found this personally helps stop me from feeling guilty or feeling like I'm making up excuses. It's to the point, formal enough to prevent miscommunication and stops me from rambling and saying sorry over and over.