Vacances[Motiu] is a list of values. “D’empresa” is a single value. Even if the list comprises only a single value, it’s a list and that can’t equal any single value. Try using the [IN](AppSheet function list - AppSheet Help function.
My prior response was simply addressing the AppSheet error message you reported. I don’t know what function you need to return the values you want since I’m not familiar with your data.
Here’s another potential pitfall:
IN("D'empresa",Vacances[Motiu]) evaluates whether the value “D’empresa” is included anywhere in the Vacances table’s entire [Motiu] column across all rows. That might not be what you want.
If you want to evaluate row by row whether the [Motiu] column equals the value “D’empresa”, then just use that comparison without any function: [Motiu] = "D'empresa".