Cannot open or create a session

[EvMo]

Mac Mini (Late 2014)
2.8 GHz Intel Core i5
OS 10.11.5
No Digidesign or M-audio hardware
Pro Tools 12.3.1
8 GB 1600 MHz DDR3
Glyph Studio mini 500 GB external drive connected via USB 3.0

I am a Pro tools novice.... no, cretin really! This is all Greek to me, but I desperately need to check some mixes before uploading for mastering. When I try to create a new session, the following error box pops up:

"Could not create a new document because Assertion in '/Volumes/Development/158347/ProTools/DFW/Views/UMenu.MacOS.mm', line 1624."

When I try to open an existing 32 bit / 88.2 kHz session, the following error message pops up:

"The current playback engine does not support a sample rate of 48 kHz."

I am not Mac or PT savvy enough to troubleshoot this and the Computer Optimization and Troubleshooting Guides page will not load. Thanks in advance for any help.