I guess you specified input frequency for input 1 and input 2. In this case, you can change the rf input source to be something between input source 1 and source 2.

For example,if you already set 2G as input source 1 in pac form, and 2.0025G as source 2, You can set PSS beat frequency as 2.001G. The error message will be gone.

However, the more natural way to do rapid IP2 for LNA is to use AC specialized analysis. IN this way, you just need to set DC in rf input port, then input 1 as 2G, input 2 2.0025G, the frequency of linear output is 2.0025G and IM output is 2.5M.