Why I can not upgrade my MacBook Pro 13-inch Early 2015 to MacOS Catalina?

PS:

And by looking to MacTracker across wide spread of hardware,

the actual device is older MacBook Pro (13-inch, Late 2011) and

somehow the name and details of About this Mac are incorrect!

The item identifies best with:

Introduced October 2011

Discontinued June 2012

Model Identifier MacBookPro8,1

Model Number A1278

EMC 2555

Memory Slots 2 - 204-pin PC-10600 (1333 MHz) DDR3 SO-DIMM

..So the last macOS that can be run there, is High Sierra 10.13.6..

Good luck and in any event, stick to that last macOS!🌞

Newest compatible operating system for that build model/year if not for Graphics, should be Catalina;

given the Graphics, looks to be High Sierra 10.13.6. (Also noted upgrade issues: may stick to High Sierra.)

Catalina is supported on MacBook Pros Mid 2012 or newer. The following has instructions that may help: macOS - How to Upgrade - Apple

Your model is listed in the following and I don't see any disclaimer or requirement about the graphics. macOS Catalina is compatible with these computers - Apple Support

A 13-inch Mid 2015 should have Intel Iris 6100 graphics. The last MBP with Intel HD Graphics 3000 was the 13-inch 2011 model. Can you do "About this Mac" to verify your sub-model?