Can I apply this ( white ) coating over my roof shingles ?
???

you have to understand the ACTUAL waterproffing of a shingle roof is
really the tar paper underneath. The shingles help protect the tar
paper or rubber coating underneath the shingles.

if your shingle roof is already leaiing or detoriating no doubt the tar
paper underneath iis bad too.

bad shingles means new roof. most shingles go 25 or 30 years so there
are other issues like flashing that most likely are due for replacement

the other issues is that shingles lose gravel, so even a perfect
coating over bad shingles will lose more gravel starting new leaks.