By "sentence," you probably mean an independent clause. Only a coordinating conjunction such as "and" or "but" can join independent clauses, and this sentence does not contain any of those.
However, there is no problem, because there is only one independent clause here:
The West Bengal government has also informed various institutes
The clauses that follow are dependent. It might be difficult to see them clearly because one contains the other. So I will show them separately:
- that . . . they will not entertain any complaints regarding any negative effect on the intern’s ongoing education
- once the NOC has been issued
#1 is a noun clause. It is the direct object of the verb informed in the independent clause.
#2 is an adverb clause. The word once is a subordinating conjunction in this sentence, not a counting word. Its meaning is very close to when.