Mapping files are essential to the core-metrics pipelines, because these pipelines actually perform a series of commands that do use metadata, e.g., for statistical comparisons.
If you only want measures of alpha diversity (e.g., richness) for each sample, and do not want to produce PCoA plots or any other information, then you can run those commands individually instead of using the core-metrics pipeline. You can use qiime diversity alpha to calculate alpha diversity (and you should really use qiime feature-table rarefy to rarefy samples at an even sequence depth prior to doing this — that's one of the steps performed in the core-metrics pipeline).
That error appears to be because you have spaces in your metadata file name. The computer is interpreting these as separate arguments (which are also separated by spaces), and you need to either (a) don't use spaces in any file names! or (b) use a backslash character before each space to "escape" the space, so that the computer interprets this command correctly.
I hope that helps!