The code you are showing works just fine, maybe you forgot to load the libraries? Try restating your R session and run your code again, including the library calls.
library(dplyr)
library(tidyquant)
(stocksassigned <- c("TSLA","GME","MKT") %>%
tq_get(get = "stock.prices", from = "2000-01-01") %>%
select(symbol, date, adjusted))
#> # A tibble: 11,979 × 3
#> symbol date adjusted
#> <chr> <date> <dbl>
#> 1 TSLA 2010-06-29 4.78
#> 2 TSLA 2010-06-30 4.77
#> 3 TSLA 2010-07-01 4.39
#> 4 TSLA 2010-07-02 3.84
#> 5 TSLA 2010-07-06 3.22
#> 6 TSLA 2010-07-07 3.16
#> 7 TSLA 2010-07-08 3.49
#> 8 TSLA 2010-07-09 3.48
#> 9 TSLA 2010-07-12 3.41
#> 10 TSLA 2010-07-13 3.63
#> # … with 11,969 more rows
Created on 2022-05-31 by the reprex package (v2.0.1)
Note: Next time please provide a proper REPRoducible EXample (reprex) illustrating your issue.