This is dependent on the jurisdiction, you cannot ask such a question without telling what country/State/Province you're at.
In the USA, in many States licensed professionals providing personal services (like lawyers, CPA's, doctors, architects, civil engineers, cosmetologists, etc) are barred from working under LLC. In some States they must operate either as sole proprietors/partnerships or as corporations (for example, California), other States provide specific legal structures like PLLC and/or PC (professional corporation) which these professionals must use. Generally this is done to disallow the "limited" liability protection for personal services.
You should consult a legal professional whether or not you should use LLC or PLLC. If you're not a State-licensed professional, usually you wouldn't be limited in using LLC (for example, EA, licensed by the Federal Government, can operate under LLC in California). This depends on the specific State you're in.