test their ability to do the job by sending them to new customers along side with a senior staff who has experience in marketing by so doing he/she would learn from his elder and also save the company time and reputation for cases of referral bases
To anticipate what frustrations you may have you must first understand what needs you are classifying as "marketing" needs. There are 8 divisions to marketing. Almost no one in marketing covers more than 2 of those divisions successfully. Marketing is a very big bucket, and if you hire someone who has natural talent in areas you don't need, they may be skilled, even as a junior career person, but they will be a very bad match for you.
Every marketer at every stage of their career is lacking competency in at least 6 of the 8 marketing skills. That's normal. It's the same way you do not hire an intellectual property attorney to represent you in a criminal case. There are plenty of inexperienced marketers that have talent in their specific areas. It's your job not to set unreasonable expectations that one marketer will solve all marketing problems.
Issues related to inexperience generally come out when you get into validation testing. An experienced marketer will have enough practice to know at a gut level whether an idea has legs. They will also know what kind of testing will be necessary to validate the assumptions they've made before wasting company money. A junior marketer will also lack a network of peers who would otherwise be contributing to refinement of their ideas, finding the best deals, etc.
You will spend more money with a junior marketer on everything else but salary, because you will have fewer precision choices that limit waste, either wasted effort, wasted time, or other mistakes. You need to know what areas you're looking for a marketer to cover to understand the likely amount of waste that comes from inexpert hands managing those areas.
So, if you can describe for me which functions you want your marketing person to perform, I can help you understand whether you will ever find this combination of skills in just one person, which skills to concentrate on for internal resources and which to hire out, and what areas to be mindful of in predicting waste.
Motivation is key. It's an entry level job so they would need to be inspired and mentored or else they might not be motivated enough
If they don't have enough experience, is the ability to "dance" between their domain knowledge (writing, editing, promotion, social management) and the business or technical goals of the management. Usually newbies either stick too much to their comfort zone knowledge or shoot too fast and miss. Take time to teach and guide them in the business goals.
"What are a hiring managers' biggest frustrations when hiring entry-level marketers? What skills or competencies do they lack?"
Failure as a result of inexperienced personnel. After all "how hard can that be?"
I can't tell you because I am not a hiring manager. But any entry level position has its issues. Better jobs come to the hire. It may be a stepping stone. Most marketing is poorly done anyway and if done correctly it is quite tedious and boring. Training is often an issue as well. Either it is too hands on or too vague. Most HR people do not inquire much about career paths. They simply ask a question "where do you see yourself in 2-5 years?" I have both career path planning services as well as marketing training programs. If interested just e-mail me firstname.lastname@example.org
This is a too generic of a question. You have to take a different approach with different type of marketers. It would be helpful if you could provide what kind of marketers you're hiring. For example: social media, email, SEO, content producer, analyst or sales. Some of them can be done by the same person but a lot require a completely different skillet and personality and a different way of measuring their effectiveness.