Cannot reopen file

get message that the file may not be sketchup fileSite Plan.skp (1.2 MB)

The first line of the SKP file, when opened in a plain-text editor, says
vp6r.oöjè SoftPlan 15.2.5 PRJ ¹ œ= a ENTRIES 0
This suggests that it is not a SketchUp SKP file at all…
I’m no expert in “SoftPlan”.
That app probably offers export options.
If SKP is one of them, then you need to get and import that into your SketchUp model.
Otherwise if another file format is available - e.g. DWG - then you can get that and import it into your SketchUp Pro model…