Welcome To HaX4Us
And it is obvious that you will try to access sdcard or external storage in termux to save your any important thing & one more noticeable thing is that the permission denied error does not come in Android 5 or before.
It means that you are here because you have Android 6.0 or later. So the only thing what you have to do is that
- run this command in termux termux-setup-storage
- After then you will be prompt for permission like as below screenshot so you have to click on allow