How to protect an outdoor electrical cable which can’t be buried?
Was this guy an actual licensed electrician?  Was there a permit pulled for this work?  Did the building inspector sign-off on the work?
Of these questions I suspect the answer is &quot;no&quot; to all of them!  Even if #1 is &quot;yes&quot; clearly he's incompetent and/or lazy.
Anyway, what you have is unacceptable and the wire should be protected from the elements and from damage.  Simply laying it on the ground, even with some type of warning tape is not going to change that.
The line needs to be buried but before doing that you need to determine if the wire used is rated for direct burial.  If should have a &quot;UF&quot; designation on it.  UF wire is more expensive than non-UF types and that's what concerns me regarding your &quot;electrician&quot;.  He cut corners on the installation, he may have cut corners on the materials as well.
Also, this type of circuit (i.e. outdoor) needs to have a GFCI breaker installed.  Where did the hookup to existing power happen?  Did he run a new circuit from the panel or did he just splice into an existing circuit?
In my opinion a license professional electrician would NEVER do such shoddy and incomplete work.  Hopefully you didn't pay this clown yet.