This is one of the common errors in UFT. There are different scenarios where this error will occur. Below is one if the scenarios.
When you are trying to call a function name without using “Call” keyword in front of the function name, then in such case “cannot use parenthesis when calling a sub “ error will occur.
For example if you call a function name like
ClickonButton(Browser(“OrangeHRM”).Page(“OrangeHRM”).WebButton(“html id:= btnSave”),”Click on Save Button”)
Then cannot use parenthesis when calling a sub error will occur as shown in below picture.

Solution:
To fix above error you need to write function name along with “Call” keyword as shown below.
Call ClickonButton(Browser(“OrangeHRM”).Page(“OrangeHRM”).WebButton(“html id:= btnSave”),”Click on Save Button”)
OR
Call FunctionName “ ParanthesisValue”