Hi @jsb81 ,
Do you enable "Allow Multiple selections" option for the Usual Days (Choice column) in your SP list?
Based on the error message that you mentioned, I think there is something wrong with the formula you typed within the Update property of the Usual Days Data card.
I assume that you have enabled "Allow Multiple selections" option for the Usual Days (Choice column) in your SP list, I have made a test on my side, please take a try with the following workaround:
The Usual Days column in my SP List as below:
The App's configuration as below:

Set the Update property of the Usual Days Data card within the Edit form to following:
ForAll(
Split(Concatenate(
If(MondayCheckbox.Value, MondayCheckbox.Text & ",", ""),
If(TuesdayCheckbox.Value, TuesdayCheckbox.Text & ",", ""),
If(WednesdayCheckbox.Value, WednesdayCheckbox.Text & ",", ""),
If(ThursdayCheckbox.Value, ThursdayCheckbox.Text & ",", ""),
If(FridayCheckbox.Value, FridayCheckbox.Text & ",", ""),
If(SaturdayCheckbox.Value, SaturdayCheckbox.Text & ",", ""),
If(SundayCheckbox.Value, SundayCheckbox.Text, "")
),
","
),
{Value: Result}
)
Set the Default property of the MondayCheckbox to following:
If(MondayCheckbox.Text in ThisItem.'Usual Days'.Value, true, false)
Set the Default property of the TuesdayCheckbox to following:
If(TuesdayCheckbox.Text in ThisItem.'Usual Days'.Value, true, false)
Set the Default property of the WednesdayCheckbox to following:
If(WednesdayCheckbox.Text in ThisItem.'Usual Days'.Value, true, false)
Set the Default property of the ThursdayCheckbox to following:
If(ThursdayCheckbox.Text in ThisItem.'Usual Days'.Value, true, false)
Set the Default property of the FridayCheckbox to following:
If(FridayCheckbox.Text in ThisItem.'Usual Days'.Value, true, false)
Set the Default property of the SaturdayCheckbox to following:
If(SaturdayCheckbox.Text in ThisItem.'Usual Days'.Value, true, false)
Set the Default property of the SundayCheckbox to following:
If(SundayCheckbox.Text in ThisItem.'Usual Days'.Value, true, false)
Please check the GIF screenshot for more details:
Please take a try with above solution, then check if the issue is solved.
Best regards,