I've spotted this on the self-service terminal in a French fast food restaurant:
What is the exact connotation of this? Does that mean a silly excuse for not being at work today?
As Alone-zee already commented, j'ai tennis is based on a well known humorous lame excuse to tell you won't be able to attend something.
The phrase means you have a planned tennis match or training.
The original sentence was Nan, j'peux pas, j'ai piscine (Nah, I can't, I have a swimming lesson), a running joke by Laurent Baffie muppet in Les Guignols de l'info in the nineties (Nulle part ailleurs - Canal+).
Nowadays, piscine can be replaced by any activity, whether sports related or not.
In that particular case, the fast-food restaurant human interface designer joke was possibly inspired by a Fédération Française de Tennis advertising campaign.
See also: J’ai bien compris qu’il ne s’agissait que d’une seule journée de tournage, seulement voilà, ce jour-là, j'ai dentiste (Blanche Gardin, 20 april 2023)
An English equivalent is Sorry, but I'm washing my hair that night
While @Alone-zee is probably correct in that the specific cultural reference, it's a construct that could literally translated to:
I have tennis
Which can be understood as either one going to play tennis or going to a tennis class but in any case, it's clear that the subject is going to play tennis.
Here, the second dimension to this is that there is a form of rejection using the pretext of 'having tennis'.
Finally, in this specific context, it seems that the device is experiencing issues and the software is displaying this message as a multi-layered joke.