Sometimes when creating a new field the error message: field name is invalid appears. There are some limitation to creating a field name.
These are the limitation for creating a field name:
1) The field name cannot be empty.
2) The field name cannot be more than 50 characters.
3) The field name cannot start with a number (e.g. 123 Field is not allowed; however, Field 123 is allowed).
4) Specials characters will not be allowed a space is allowed. (e.g. special characters are; */+-$# etc.)