Lens won’t go on canon eos r6

[Gia]

I got a brand new canon eos r6 and lenses to go along with it but one of my lenses won’t go on while the other works fine. How can I get it on? It’s the canon zoom lens ef 75-300mm 1:4-5.6

[shadowsports]

Greetings,

Canon (mirrorless) uses an RF mount.  You can use your EF lens with the R6, but you'll need an adapter.  

They make 2

Mount Adapters | Mount Adapter EF-EOS R | Canon USA

Mount Adapters | Control Ring Mount Adapter EF-EOS R | Canon USA

[Tronhard]

The R6 is an awesome camera, the EF 75-300 lens is nowhere near that quality. I strongly recommend replacing it and getting either:
RF24-240 lens - no adapter needed, and OIS works with IBIS see a review with video HERE 
RF 100-400 If you want to stick with at least the same zoom range: review and video HERE 
If you prefer to stick the the EF mount, and get an adapter, then check out the EF 70-300 range (which you can likely pick up relatively cheaply) see my own reviews HERE