This error occurs when an integral is not formed correctly.  Integrals that are entered with the $\int$ symbol must end with a differential; however, simply typing 'dx' is not sufficient.  Maple requires a special character for the 'd' prefix of the integration variable (DifferentialD).

To correctly enter an integral in 2-D math, do one of the following:

Using these methods ensures your expression is parsable Maple syntax.

Integrals can also be entered using the int command.

Tip: If an expression is executed accidentally, it can lead to error messages (if it is not valid Maple syntax). If this happens, toggling the expression to nonexecutable math removes the error message and changes the math to nonexecutable. To change an expression to nonexecutable math use the shortcut key Shift + F5.  For more information, see Executable and Nonexecutable Math.

Example 1. This integral is missing the differential.

Solution - Use command completion

Type int and press Esc.  From the pop-up list, select the desired template.  

In this case we select int (indefinite).  It is inserted with the placeholder f selected.  Type sin(x).  Press Tab to go to the next placeholder, and enter x for the variable of integration.

Example 2. This integral was formed by using the $\int$ symbol then typing sin(x)dx.  The dx is not recognized as a differential.

Solution - Use the palettes 

From the Calculus palette, select the template $\int f\ⅆx$.  Replace the placeholder $f$  with $\sin \left(x\right)$.  Use the Tab key to move to the next placeholder. Replace the placeholder $x$ with $x$.  Execute.

Solution - Use the int command

You can also enter an integral using the int command.

Note: In Maple 2019 and earlier versions, error message that occurs in these examples is slightly different:

The solutions to the problem are the same as described above.