Hi @robmerritt ,
Firstly you have enabled the Power Fx while creating a flow. Power FX in PAD is still in preview state,so we dont recommend it to use in production or development.Please disable the Power Fx as shown in below image.

Once disabled create a new flow with Power Fx diabled and Try to paste the code provided below.Please find the solution.

Code:
Please copy this code to your flow.
DateTime.GetCurrentDateTime.Local DateTimeFormat: DateTime.DateTimeFormat.DateAndTime CurrentDateTime=> CurrentTime
Text.ConvertDateTimeToText.FromDateTime DateTime: CurrentTime StandardFormat: Text.WellKnownDateTimeFormat.LongTime Result=> CurrentTime
Text.ConvertTextToDateTime.ToDateTimeCustomFormat Text: CurrentTime CustomFormat: $'''HH:mm:ss''' DateTime=> CurrentTime
SET EndTime TO $'''10:58:00'''
Text.ConvertTextToDateTime.ToDateTimeCustomFormat Text: EndTime CustomFormat: $'''HH:mm:ss''' DateTime=> EndTime
LOOP WHILE (CurrentTime) <= (EndTime)
Display.ShowMessageDialog.ShowMessageWithTimeout Message: $'''Test''' Icon: Display.Icon.None Buttons: Display.Buttons.OK DefaultButton: Display.DefaultButton.Button1 IsTopMost: False Timeout: 1 ButtonPressed=> ButtonPressed13
DateTime.GetCurrentDateTime.Local DateTimeFormat: DateTime.DateTimeFormat.DateAndTime CurrentDateTime=> CurrentTime
Text.ConvertDateTimeToText.FromDateTime DateTime: CurrentTime StandardFormat: Text.WellKnownDateTimeFormat.LongTime Result=> CurrentTime
END
(Note:- if you got your solution you can mark as solution and gives kudos)
Thanks & Regards
Vishnu Reddy