Contractor converted bathtub to shower, finished yesterday, shower has standing water at back and middle of shower after use. What are possible solutions.
The short answer
As others have said in comments, the slope of the shower - called the "fall" - is the most fundamental thing to get right. You need a gentle fall from the edges of the shower to the drain on all four sides, so that any water immediately flows to the drain. This is "bathroom installation 101", and an installer who does not know this should not be licenced to operate.
I'm sure you won't want to hear this, but the solution is likely to require a contractor removing the shower, ripping up the tiles, re-levelling the floor to apply the correct fall, re-tiling and then reinstalling the shower. Worse, if the contractor was really dodgy, you may find that the waterproofing on the walls & floor is substandard, the implications of which are serious.
It's essential that you do not pay the contractor (if you haven't done so already) and that you let them know that you will take legal action if they do not fix it. The bad news is that cost of the rework will now be several times the cost of original work.
The longer answer
From a "doing" point of view, this is what needs to happen if it is done properly.
If the shower frame was installed after the tiles went down (as would be the case with a frameless shower), then the glass and frame need to be taken out and put gently into another room.
Next, all the tiles on the shower floor need to be chiselled out. Worse, if the wall tiles were laid down after the floor tiles, then the bottom one or two rows of wall tiles need to come off before the floor tiles.
Then the precise edges of the shower frame need to be marked perfectly on the floor, so there is no ambiguity about precisely where the high points need to be.
Next, a competent floor-leveller needs to use re-level the floor under the shower, so that it falls correctly down to the drain on all sides. This could end up an even bigger job, because if the level of the shower is ends up higher than the surrounding bathroom and there is no drain in the rest of the bathroom, then the existing ardit may need to be jack-hammered up before new ardit is laid.
Then the floor tiles need to be re-laid. Depending on how well the ardit work was done at the previous step, there may also be a need to add some extra slope with the tiles (e.g. by adding thicker blobs of glue to the outside edges of the tiles near the frame and less glue immediately near the drain).
Finally, the wall tiles and shower need to be put back.
I note further that an installer who hasn't installed the correct slope may also have taken short-cuts in other areas. For instance, were the shower walls and floor fully water-proofed?
If not, you have an even bigger problem on your hands. A bath is obviously water-proof, but a shower is not: grout is porous and therefore water will seep into the walls and floor every time you shower. The problem will be especially bad if water is pooling on the shower tiles instead of flowing down the drain, because it will soak through the grout and then into the sub-floor and eventually into other rooms as well.
Notwithstanding all the bad news above, depending on what exactly what done by the contractor, there may be short-cuts to fix the problem. As someone suggested in the comments, more information about the installation would help us answer your problem.
Please note that I'm not a builder, just a person who has done renovations myself (including a very recent complete bathroom demolition & rebuild).