unable to add lines to buil prop

I have the lg g5 t mobile as the title says it im unable to add lines to build prop. My phone is stock rooted tried with root browser and build prop editor both of which fails. I cant even seem to type anything in the build prop file and the only way i can type in the file is if i delete a line to replace my own build prop stuff.
I have tried with both 10A rom and 10D no improvements

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