error in data.frame

ok, does that function call run without errors ?
what change does it make in your environment ?
The documentation (https://rdrr.io/cran/activpalProcessing/man/process.AP.html) says that it would create 3 csv files. Do you see them having been written out ?

This is where the error appears.

[1] 1001
[1] "1"
[1] "SBIntervention"
[1] 1002
[1] "1"
[1] "SBIntervention"
[1] 1003
[1] "1"
[1] "SBIntervention"
[1] 1004
[1] "1"
[1] "SBIntervention"
[1] 1005
[1] "1"
[1] "SBIntervention"
[1] 1006
[1] "1"
[1] "SBIntervention"
[1] 1007
[1] "1"
[1] "SBIntervention"
[1] 1008
[1] "1"
[1] "SBIntervention"
[1] 1101
[1] "1"
[1] "SBIntervention"
[1] 1102
[1] "1"
[1] "SBIntervention"
Error in data.frame(study = study, sub = as.numeric(s), visit = v, date = unique(data$date), :
arguments imply differing number of rows: 1, 0, 37379
In addition: Warning message:
In activpal.file.reader(file.name.and.path) : NAs introduced by coercion

yes it starts writing 3 files out - means table, results table and a sleep wear wake table. It writes up to participant 1101 then stops

ok, given the circumstance that this is a function that is part of this activpalProcessing package, you probably need an expert in activpalProcessing. I don't have the resources to become an expert by studying it to find the issue.
Perhaps contact the package creator, or look for a community of users to ask.

https://cran.r-project.org/web/packages/activpalProcessing/index.html
note their contact listed

Thank you for all of your help