You have picked the ideal screen position, but the expected stud is not there. That does not tell you which fastener to use next. First establish the wall construction and the installation methods permitted by the chosen mount.
Do not assume every mount accepts wall anchors
A bracket designed for structural attachment cannot be made suitable for drywall alone merely by choosing an anchor with an impressive label. The full assembly, wall condition and installation instructions matter.
Some specialized products do allow a particular drywall-only method under strict conditions. The SANUS HLL1 manual is one example, with specific requirements and exclusions. It is not a general approval for other mounts, patched walls or different construction.
Reconsider the screen position and equipment
Ask whether a different compatible wall plate can reach suitable attachment points while keeping the screen near the preferred position. Another option may be a different wall or a properly assessed structural solution. Avoid deciding on hardware before those alternatives have been considered.
Metal framing, plaster over lath and masonry behind a finish can complicate the picture. If a detector gives inconsistent results, do not turn uncertainty into a series of exploratory holes without understanding what might be inside the wall.
What to send for an assessment
- The TV and mount model numbers.
- A photo of the entire intended area.
- Any known information about the wall construction.
- Whether the screen must be centered at one exact point.
- Whether a full-motion or fixed position is needed.
The right outcome may be a revised plan rather than an immediate installation. That is better than concealing an unsuitable attachment behind the TV and assuming the result is secure.