Why I can't install new apps and delete old photos?

[Windows Central Question]

I've a Lumia 730 equipped with a 32 GB SD card. I've a lot of videos and photos in it and there is only 2.3 GB space free now. Recently I'm facing some problems on my Lumia 730. While trying to install new application it says "Attention Required. Tap here". After doing it a window pops up suggesting to update the software. When I try to update the software it detects that my phone is up-to-date. I thought that may be it was caused by the low disc space on my SD card. So i tried to delete my old pics by browsing through PC and also by selecting files from the phone itself. But they don't get deleted. Also I've found that I can't store my newly taken photos on the phone. It shows the photos for sometime like 10-15 mins. But after that those pics get lost. What is the problem and what I should do to solve it? Thanks in advance!!!