I am trying to understand what you are hoping to do in that OnSelect property. You mentioned a "statement" "CalMonthCurentDate.Text = 1 " -- are you trying to force a value into a property of a control, imperatively? (i.e. "set" the property?)
If that is what you are trying to do, PowerApps does not work that way. The "=" operator is an equality operator, not an assignment operator. Unlike imperative languages like JS, C++, Java, C#, etc. where you tell objects what to do, PowerApps's expression language is a declarative dataflow language, in which you declare what data flows into each property, and how it needs to be transformed on its way into the property. This is very similar to how Excel formulas work: you declare how each cell's value needs to be computed; and the dataflow engine does the rest of the work, iteratively flowing data where it needs to go, and evaluating all affected formulas to a steady state.
If the goal is to set the Text property of CalMonthCurrentDate to "1", you will need to declare that value ("1") as the default value flowing into the control. For example for a Textbox control, bind its Text property to "1". For a TextInput control, bind its Default property to "1". If you are dealing with a stateful control, such as a TextInput control, and you want to "reset it" to some value such as "1" from an OnSelect property of another control, you need to pulse the Default property from that formula:
TextInput1.Default = DefaultValue
Button123.OnSelect = UpdateContext({DefaultValue: ""}); UpdateContext({DefaultValue: "1"})
Please see the formula reference for more information.
https://powerapps.microsoft.com/en-us/tutorials/formula-reference/
I hope this helps.
