The negative 91 is showing radar elevation, and -91 isn't going to work.
The top number shows the upper limit of the radar cone in thousands of feet (altitude) at the distance of the cursors. The bottom number shows the lower limit of the cone. Push it further away and the numbers diverge because of the expanding cone. Pull the cursors closer and the numbers converge because the cone is narrower at closer ranges
So the goal is to get the cursors with an altitude range that includes the altitude the target is flying at. I think radar elevation work is one of the things new players struggle the most with.
What do you have radar elevation set to? Commonly a rotary or other analog input is used as it's an assignable axis. The Cougar for example has the radar elevation rotary which makes it easy. If your stick has an analog control you can use for this it is best.
In your case your radar cone is looking almost straight down, and is not scanning the airspace the Fishbeds are flying in. You need to tilt that cone upwards to scan this airspace and then can attain a lock.
TMS Up will lock the target. But what it is set to depends on the key file you are using. Check the keyfile to see what it is. When you are able to paint that return with your radar elevation work, slew the cursors over the return and release TMS Up and it will lock the target. At that point it is as good as dead