In Maple, you can use almost any letter as as a variable, even when that letter is also used to represent a particular mathematical symbol or constant, such as the exponential constant $\ⅇ\,$ the differential symbol $\ⅆ$, or the imaginary unit $\mathrm{i}$.  To achieve this flexibility, these characters need to be entered as special symbols.  

This warning occurs if you type something that resembles an expression with the exponential $\ⅇ$, but use a variable $e$, not the exponential symbol $\ⅇ$. In order to enter the exponential function, use command/symbol completion, the palettes, or the exp function, as demonstrated in the examples below.

Note the difference in typesetting: the variable $e$ is displayed in italics, whereas the exponential constant is displayed without italics: $\ⅇ\.$ Using the wrong one leads to unexpected results, and often, this warning.

Variable $\mathit{e}$:

Symbol $\mathbf{\ⅇ}\mathbf{\:}$

If in fact you did want to use e as simply a variable and you do not want to see this message, you can turn off this and similar warning messages by using Typesetting:-Settings(parserwarnings=false).  See Example 2.

Example 1

Typing e by hand does not create an expression meaning the base of the natural logarithm ($\ⅇ \approx 2.718231828$ ).  Instead, the e is simply a variable.

Solution 1 - Use the command/symbol completion

Use command/symbol completion to enter the exponential constant $\ⅇ$.  Start by typing e, then press Esc.  Select Exponential 'e' from the pop-up list.

Solution 2 - Use the Palettes

The Expression palette provide templates for entering mathematical expressions.  It's useful when you want to write a power of $\ⅇ$.  Alternatively, the Common Symbols palette has an entry for $\ⅇ$ which you can incorporate into a larger expression.

To use the Expression palette, place your cursor in your document, then click the   template.  The placeholder is highlighted.  Type x.  Press enter to execute.

Solution 3 - The exp Function

You can also use the exp function:

See the exp help page for additional information.

Example 2 - Prevent this warning from appearing

Suppose do you want to use e as a name (a variable), and did not intend it to mean the exponential e.

Solution

You can control whether parse warning messages appear via Typesetting:-Settings.

Now you do not see the warning message.