Garage door won’t open unless I hold the wall button down

The blinking red light on the sender would indicate that they are mis-aligned. The best way to test this is to remove them from the mounting brackets and hold them a couple of inches apart with the sensors aligned. Have somebody hit the door closer, then point them away from each other. If they work as expected that way, you'll just have to fiddle around with the mounts until you have them correctly aligned.

If both sensor lights are not on, the door opener will not close and the light will blink because the opener thinks there is something in the door path. This is a safety measure. One light being off is usually due to either the two sensors getting out of alignment, a faulty connection, or a faulty sensor. You say that you have installed new door sensors. If both lights are still not on, you have to troubleshoot that first. Check all connections and then try to align the 2 sensors. If both green lights are on, and the opener will still not open, then the fault is probably elsewhere.