Alright it worked!
Thank you, I'm planning on reading all the other SFML books at some point and learn better practices, but for now I'm satisfied with the results.
The derived player class also works fine and I removed the texture member and instead called the asset manager directly as an argument:
entity_sprite = spr.set_sprite(AssetManager::LoadTexture("res/wildcard.png"), { 0,0,36,63 }, { 36,63 });
I'm gonna go and update the SFML library when I feel like it, but for now, thank you both, your help was invaluable, seriously, thank you so much!