You have to first permit installation from unknown sources....
Then you have to make sure your DEVICE INTERNAL STORAGE is not taken up by too much media and too many downloads. Internal Storage (phone memory) space is where all the APK files are installed.
If you have less than 5 GB of internal storage available, you need to either delete some files, uninstall apps you don't use, or transfer downloads and media to your external storage (SD card in your phone).
If you have not updated your Android OS version to 15, your APK max allowed size is 2.5 GB.
In Android OS 15, that was expanded to 10 GB per APK.
Lewd Town Adventures APK is 3 GB, which is why you need the space to install + the OS 15 update to allow that large of an APK.