I think I can safely say that I am stuck. I was following a youtube tutorial at the following link
I got stuck somewhere in the 20 - 22 minute mark. I am trying to connect colors (as parameters to a LERP). Below is a picture of the setup and the error message. As soon as I disconnect the links between the top leftmost LERP, the problem is fixed. The color parameters are the same as all the others (I pressed 4+ left click).
To put it simply, some nodes treat color output as RGB while others do RGBA, so when you have that error you must first find which node causes that, and then either use append vector node and use float of 1, that way you will turn RGB into RGBA (vec3 into vec4). Or the other way around, use component mask node to grab only RGB from RGBA. As far as offenders, they are all over the place. It could be something as simple as some blending mode that suddenly does not internally work on alpha and outputs RGB or vice versa. Happens a lot.
In your case start by using RGBA output at the very bottom on those nodes first. Maybe will fix it.