It is very clear that the Employer’s fulfilling its obligations is a must for the performance of SAT4 test since it is impossible to carry out SAT4 activities without having IC and freight trains.
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While it is grammatically correct, it's a bit "clunky". I would suggest revising to:
It is very clear that the Employer must fulfill its obligations in order for the SAT4 test to be performed since it is impossible to carry out SAT4 activities without having IC and freight trains.
Yes, it's grammatically correct.
The phrase "fulfilling its obligations" acts like a noun here, so imagine putting some other noun in its place and it will be more clear that the structure is fine.
Here's an example of that same sentence without all the phrases:
"It is clear that the employer's [clown] is a must for the performance of [balloon tricks] since it is impossible to carry out [balloon tricks] without [the balloons a clown would bring].