I can find no listing on Amazon re which Roku models Amazon has written their Prime Video Roku channel app to run on. However, Amazon announced in emails to Roku owners that, effective November 1, 2021, their Prime Video would no longer support the following older Roku model numbers:
Roku 2 HD/XD/XS models 2050X, 2500X, 2720X, 3000X, 3050X, 3100X Roku LT models 2400X, 2450X, 2700X Roku SE model 2710X Roku Streaming Stick models 3400X, 3420X, 3500X
(The Xs or other letters at the end of the model number are distribution codes and are irrelevant for this issue.)
You can find your Roku model number via Roku menu path Settings / System / About
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X