Contrary to most other languages, Ada accept the underscore (_) character within identifiers but not at the start, nor at the end. This allows your compiler to use such underscores and not clash with user identifiers (i.e. _strlen() would always be a compiler intrinsic, a user could only write an strlen() function.)
To fix the problem, change your identifier and remove the starting and/or ending underscores. Also, make sure you don't use two underscores one after another.
Note that numbers can also include underscores and they have the same limit. However, underscores in numbers are ignored in the final result (i.e. 1_000 is taken as 1000.)