Your first attempt isn't a formula, so saying that Temperatuur = 15 C, it then should be assigning the value:

Text(${Temperatuur}) //Text(15 C)

I get the expected output from:

=Text(Temperatuur) //15 C

While using:

=Text(${Temperatuur})

I get these errors regarding unexpected characters:

Parameter 'Value': Unexpected characters. Characters are used in the formula in an unexpected way.
Unexpected characters. The formula contains 'CurlyOpen' where 'ParenClose' is expected.
Unexpected characters. Characters are used in the formula in an unexpected way.
The function 'Text' has some invalid arguments.
Expected text or number. We expect text or a number at this point in the formula.

Best regards,

Gustav